Adjusting the colour of various parts of the UI is a fun way to get started with Custom CSS.
Unfortunately, due to the way that Notable applies its own styles, you have to use !important whenever you want to change the colour of something - this includes background colours. On that note, be sure to use the background-color property over the shorthand background property so that the background colour is overridden correctly.
You may also need to use !important to override some other properties. Use the Styles pane in DevTools to check if that is the case.