Tweaks to Onboarding following design review.

Use constants for consistent top padding and icon sizes.
Match layout of AnalyticsPrompt on iPad to other views.
Update icon in use case selection screen to allow tinting.

RiotSwiftUI/Modules/AnalyticsPrompt/View/AnalyticsPrompt.swift

@@ -114,17 +114,23 @@ struct AnalyticsPrompt: View {
         GeometryReader { geometry in
             VStack {
                 ScrollView(showsIndicators: false) {
+                    Spacer()
+                        .frame(height: OnboardingMetrics.spacerHeight(in: geometry))
+                    
                     mainContent
-                        .padding(.top, 50)
+                        .frame(maxWidth: OnboardingMetrics.maxContentWidth)
                         .padding(.horizontal, horizontalPadding)
+                        .padding(.top, OnboardingMetrics.breakerScreenTopPadding)
                 }
-                .frame(maxWidth: OnboardingMetrics.maxContentWidth)
                 .frame(maxWidth: .infinity)
                 
                 buttons
                     .frame(maxWidth: OnboardingMetrics.maxContentWidth)
                     .padding(.horizontal, horizontalPadding)
                     .padding(.bottom, geometry.safeAreaInsets.bottom > 0 ? 0 : 16)
+                
+                Spacer()
+                    .frame(height: OnboardingMetrics.spacerHeight(in: geometry))
             }
             .background(theme.colors.background.ignoresSafeArea())
             .accentColor(theme.colors.accent)
@@ -136,10 +142,13 @@ struct AnalyticsPrompt: View {
 
 // MARK: - Previews
 
-@available(iOS 14.0, *)
+@available(iOS 15.0, *)
 struct AnalyticsPrompt_Previews: PreviewProvider {
     static let stateRenderer = MockAnalyticsPromptScreenState.stateRenderer
     static var previews: some View {
         stateRenderer.screenGroup()
+            .theme(.light).preferredColorScheme(.light)
+        stateRenderer.screenGroup()
+            .theme(.dark).preferredColorScheme(.dark)
     }
 }