tachiyomi/app/src/main/res/menu/reader.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto">

    <item
        android:id="@+id/action_custom_filter"
        android:icon="@drawable/ic_brightness_4_white_24dp"
        android:title="@string/custom_filter"
        app:showAsAction="ifRoom" />

    <item
        android:id="@+id/action_settings"
        android:icon="@drawable/ic_settings_white_24dp"
        android:title="@string/label_settings"
        app:showAsAction="ifRoom"
        />

</menu>