tachiyomi/app/src/main/res/drawable/material_bubble_drawable.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <corners android:radius="38dp" />
    <solid android:color="?attr/colorPrimaryVariant" />
    <size
        android:width="30dp"
        android:height="30dp" />
</shape>