How to customize Pixel 4 XL theme:
Step 1: Open app drawer in your Google Pixel 4 and Pixel 4 XL.
Step 2: Tap on Display
Step 3: Tap on Styles and wallpapers
Step 4: Swipe left side and tap on Custom 1
Step 5: Choose specific font style from list and tap on Next
Step 6: Choose icons such as Wi-Fi, Bluetooth, Airplane mode, Flash light and few others.
Step 7: Tap on Next
Step 8: Choose color from list and tap on Next
Step 9: Choose icon shape on Pixel 4 XL & Pixel 4 and tap on Next
Step 10: Enter style name you customize Pixel 4 theme and tap Apply at bottom right corner.