feat: introduce provider button shape customization and update theming API

Author: demolaf

app/src/main/java/com/firebaseui/android/demo/CustomSlotsThemingDemoActivity.kt

@@ -13,12 +13,15 @@ import androidx.compose.foundation.layout.height
 import androidx.compose.material3.Button
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.ShapeDefaults
+import androidx.compose.material3.Shapes
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.Shape
 import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.unit.dp
 import com.firebase.ui.auth.AuthException
@@ -42,8 +45,13 @@ class HighLevelApiDemoActivity : ComponentActivity() {
         val authUI = FirebaseAuthUI.getInstance()
         val emailLink = intent.getStringExtra(EmailLinkConstants.EXTRA_EMAIL_LINK)
 
+        val customTheme = AuthUITheme.Default.copy(
+            providerButtonShape = ShapeDefaults.ExtraLarge
+        )
+
         val configuration = authUIConfiguration {
             context = applicationContext
+            theme = customTheme
             logo = AuthUIAsset.Resource(R.drawable.firebase_auth)
             tosUrl = "https://policies.google.com/terms"
             privacyPolicyUrl = "https://policies.google.com/privacy"

app/src/main/java/com/firebaseui/android/demo/HighLevelApiDemoActivity.kt

@@ -16,48 +16,59 @@ Equivalent FirebaseUI libraries are available for [iOS](https://github.com/fireb
 
 ## Table of Contents
 
+### Getting Started
 1. [Demo](#demo)
-1. [Setup](#setup)
-   1. [Prerequisites](#prerequisites)
-   1. [Installation](#installation)
-   1. [Provider Configuration](#provider-configuration)
-1. [Quick Start](#quick-start)
-   1. [Minimal Example](#minimal-example)
-   1. [Check Authentication State](#check-authentication-state)
-1. [Core Concepts](#core-concepts)
-   1. [FirebaseAuthUI](#firebaseauthui)
-   1. [AuthUIConfiguration](#authuiconfiguration)
-   1. [AuthFlowController](#authflowcontroller)
-   1. [AuthState](#authstate)
-1. [Authentication Methods](#authentication-methods)
-   1. [Email & Password](#email--password)
-   1. [Phone Number](#phone-number)
-   1. [Google Sign-In](#google-sign-in)
-   1. [Facebook Login](#facebook-login)
-   1. [Other OAuth Providers](#other-oauth-providers)
-   1. [Anonymous Authentication](#anonymous-authentication)
-   1. [Custom OAuth Provider](#custom-oauth-provider)
-1. [Usage Patterns](#usage-patterns)
-   1. [High-Level API (Recommended)](#high-level-api-recommended)
-   1. [Low-Level API (Advanced)](#low-level-api-advanced)
-   1. [Custom UI with Slots](#custom-ui-with-slots)
-1. [Multi-Factor Authentication](#multi-factor-authentication)
-   1. [MFA Configuration](#mfa-configuration)
-   1. [MFA Enrollment](#mfa-enrollment)
-   1. [MFA Challenge](#mfa-challenge)
-1. [Theming & Customization](#theming--customization)
-   1. [Material Theme Integration](#material-theme-integration)
-   1. [Custom Theme](#custom-theme)
-   1. [Provider Button Styling](#provider-button-styling)
-1. [Advanced Features](#advanced-features)
-   1. [Anonymous User Upgrade](#anonymous-user-upgrade)
-   1. [Email Link Sign-In](#email-link-sign-in)
-   1. [Password Validation Rules](#password-validation-rules)
-   1. [Credential Manager Integration](#credential-manager-integration)
-   1. [Sign Out & Account Deletion](#sign-out--account-deletion)
-1. [Localization](#localization)
-1. [Error Handling](#error-handling)
-1. [Migration Guide](#migration-guide)
+2. [Setup](#setup)
+   - [Prerequisites](#prerequisites)
+   - [Installation](#installation)
+   - [Provider Configuration](#provider-configuration)
+3. [Quick Start](#quick-start)
+   - [Minimal Example](#minimal-example)
+   - [Check Authentication State](#check-authentication-state)
+
+### Core Concepts
+4. [Core Concepts](#core-concepts)
+   - [FirebaseAuthUI](#firebaseauthui)
+   - [AuthUIConfiguration](#authuiconfiguration)
+   - [AuthFlowController](#authflowcontroller)
+   - [AuthState](#authstate)
+
+### Authentication
+5. [Authentication Methods](#authentication-methods)
+   - [Email & Password](#email--password)
+   - [Phone Number](#phone-number)
+   - [Google Sign-In](#google-sign-in)
+   - [Facebook Login](#facebook-login)
+   - [Other OAuth Providers](#other-oauth-providers)
+   - [Anonymous Authentication](#anonymous-authentication)
+   - [Custom OAuth Provider](#custom-oauth-provider)
+6. [Multi-Factor Authentication](#multi-factor-authentication)
+   - [MFA Configuration](#mfa-configuration)
+   - [MFA Enrollment](#mfa-enrollment)
+   - [MFA Challenge](#mfa-challenge)
+
+### Implementation
+7. [Usage Patterns](#usage-patterns)
+   - [High-Level API (Recommended)](#high-level-api-recommended)
+   - [Low-Level API (Advanced)](#low-level-api-advanced)
+   - [Custom UI with Slots](#custom-ui-with-slots)
+8. [Theming & Customization](#theming--customization)
+   - [Using Default Themes](#using-default-themes)
+   - [Customizing Default Theme](#customizing-default-theme)
+   - [Inheriting from Material Theme](#inheriting-from-material-theme)
+   - [Creating a Completely Custom Theme](#creating-a-completely-custom-theme)
+   - [Provider Button Styling](#provider-button-styling)
+
+### Advanced
+9. [Advanced Features](#advanced-features)
+   - [Anonymous User Upgrade](#anonymous-user-upgrade)
+   - [Email Link Sign-In](#email-link-sign-in)
+   - [Password Validation Rules](#password-validation-rules)
+   - [Credential Manager Integration](#credential-manager-integration)
+   - [Sign Out & Account Deletion](#sign-out--account-deletion)
+10. [Localization](#localization)
+11. [Error Handling](#error-handling)
+12. [Migration Guide](#migration-guide)
 
 ## Demo
 
@@ -970,17 +981,50 @@ fun ManualMfaChallenge(resolver: MultiFactorResolver) {
 
 ## Theming & Customization
 
-### Material Theme Integration
+FirebaseUI Auth provides flexible theming options to match your app's design:
 
-FirebaseUI automatically inherits your app's Material Theme:
+- **`AuthUITheme.Default`** / **`AuthUITheme.DefaultDark`** - Pre-configured Material Design 3 themes
+- **`.copy()`** - Customize specific properties of the default themes (data class)
+- **`fromMaterialTheme()`** - Inherit from your app's existing Material Theme
+- **Custom theme** - Full control over colors, typography, shapes, and provider button styles
+
+### Using Default Themes
+
+FirebaseUI provides pre-configured themes for light and dark modes:
+
+```kotlin
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Email(), AuthProvider.Google())
+    theme = AuthUITheme.Default  // or AuthUITheme.DefaultDark
+}
+```
+
+### Customizing Default Theme
+
+Use `.copy()` to customize specific properties of the default theme:
+
+```kotlin
+val customTheme = AuthUITheme.Default.copy(
+    providerButtonShape = MaterialTheme.shapes.extraLarge  // Pill-shaped buttons
+)
+
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Google(), AuthProvider.Email())
+    theme = customTheme
+}
+```
+
+### Inheriting from Material Theme
+
+Use `fromMaterialTheme()` to automatically inherit your app's Material Design theme:
 
 ```kotlin
 @Composable
 fun App() {
     MyAppTheme {  // Your existing Material3 theme
         val configuration = authUIConfiguration {
             providers = listOf(AuthProvider.Email())
-            theme = AuthUITheme.fromMaterialTheme()  // Inherits MyAppTheme
+            theme = AuthUITheme.fromMaterialTheme()  // Inherits colors, typography, shapes
         }
 
         FirebaseAuthScreen(
@@ -991,9 +1035,20 @@ fun App() {
 }
 ```
 
-### Custom Theme
+You can also customize while inheriting:
 
-Create a completely custom theme:
+```kotlin
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Google(), AuthProvider.Facebook())
+    theme = AuthUITheme.fromMaterialTheme(
+        providerButtonShape = RoundedCornerShape(16.dp)  // Override button shape
+    )
+}
+```
+
+### Creating a Completely Custom Theme
+
+Build a theme from scratch with full control:
 
 ```kotlin
 val customTheme = AuthUITheme(
@@ -1011,7 +1066,8 @@ val customTheme = AuthUITheme(
         small = RoundedCornerShape(4.dp),
         medium = RoundedCornerShape(8.dp),
         large = RoundedCornerShape(16.dp)
-    )
+    ),
+    providerButtonShape = RoundedCornerShape(12.dp)
 )
 
 val configuration = authUIConfiguration {
@@ -1022,31 +1078,126 @@ val configuration = authUIConfiguration {
 
 ### Provider Button Styling
 
-Customize individual provider button styling:
+#### Setting shapes for all provider buttons
+
+**Option 1: Using `.copy()` on default theme:**
+
+```kotlin
+val customTheme = AuthUITheme.Default.copy(
+    providerButtonShape = RoundedCornerShape(12.dp)  // Applies to all provider buttons
+)
+
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Google(), AuthProvider.Facebook(), AuthProvider.Email())
+    theme = customTheme
+}
+```
+
+**Option 2: Using `fromMaterialTheme()`:**
+
+```kotlin
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Google(), AuthProvider.Facebook())
+    theme = AuthUITheme.fromMaterialTheme(
+        providerButtonShape = RoundedCornerShape(16.dp)
+    )
+}
+```
+
+**Option 3: Creating custom theme:**
+
+```kotlin
+val customTheme = AuthUITheme(
+    colorScheme = MaterialTheme.colorScheme,
+    typography = MaterialTheme.typography,
+    shapes = MaterialTheme.shapes,
+    providerButtonShape = RoundedCornerShape(12.dp)
+)
+
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Google(), AuthProvider.Facebook(), AuthProvider.Email())
+    theme = customTheme
+}
+```
+
+#### Customizing individual provider buttons
+
+Customize specific provider buttons using the pre-defined `ProviderStyleDefaults` constants:
+
+**Using `.copy()` with default theme:**
 
 ```kotlin
 val customProviderStyles = mapOf(
-    "google.com" to AuthUITheme.ProviderStyle(
-        backgroundColor = Color.White,
-        contentColor = Color(0xFF757575),
-        iconTint = null,  // Use original colors
+    "google.com" to ProviderStyleDefaults.Google.copy(
         shape = RoundedCornerShape(8.dp),
         elevation = 4.dp
     ),
-    "facebook.com" to AuthUITheme.ProviderStyle(
-        backgroundColor = Color(0xFF1877F2),
-        contentColor = Color.White,
-        shape = RoundedCornerShape(12.dp),
+    "facebook.com" to ProviderStyleDefaults.Facebook.copy(
+        shape = RoundedCornerShape(24.dp),
         elevation = 0.dp
     )
 )
 
 val customTheme = AuthUITheme.Default.copy(
-    providerStyles = customProviderStyles
+    providerButtonShape = RoundedCornerShape(12.dp),  // Default for all
+    providerStyles = customProviderStyles  // Specific overrides
+)
+
+val configuration = authUIConfiguration {
+    providers = listOf(AuthProvider.Google(), AuthProvider.Facebook())
+    theme = customTheme
+}
+```
+
+**Using `fromMaterialTheme()`:**
+
+```kotlin
+val customProviderStyles = mapOf(
+    "google.com" to ProviderStyleDefaults.Google.copy(
+        shape = RoundedCornerShape(8.dp),
+        elevation = 4.dp
+    )
 )
 
 val configuration = authUIConfiguration {
     providers = listOf(AuthProvider.Google(), AuthProvider.Facebook())
+    theme = AuthUITheme.fromMaterialTheme(
+        providerButtonShape = RoundedCornerShape(12.dp),
+        providerStyles = customProviderStyles
+    )
+}
+```
+
+#### Complete customization example
+
+Real-world example combining global and per-provider customizations:
+
+```kotlin
+// Define custom styles for specific providers
+val customProviderStyles = mapOf(
+    "google.com" to ProviderStyleDefaults.Google.copy(
+        shape = RoundedCornerShape(24.dp),  // Pill-shaped Google button
+        elevation = 6.dp
+    ),
+    "facebook.com" to ProviderStyleDefaults.Facebook.copy(
+        shape = RoundedCornerShape(8.dp),  // Medium rounded Facebook button
+        elevation = 0.dp  // Flat design
+    )
+    // Email provider will use the global providerButtonShape
+)
+
+// Customize default theme with global button shape and per-provider styles
+val customTheme = AuthUITheme.Default.copy(
+    providerButtonShape = RoundedCornerShape(12.dp),  // Global default for all buttons
+    providerStyles = customProviderStyles  // Specific overrides
+)
+
+val configuration = authUIConfiguration {
+    providers = listOf(
+        AuthProvider.Google(),      // Uses custom shape (24.dp)
+        AuthProvider.Facebook(),    // Uses custom shape (8.dp)
+        AuthProvider.Email()        // Uses global shape (12.dp)
+    )
     theme = customTheme
 }
 ```