This is a great write-up of Jason
Great that you're focusing on this often overlooked part! I recommend starting with the learning path in the Mendix Academy: https://academy.mendix.com/link/paths/105/Style-your-App-with-CSS. It has some great learning to get you started on this path. If you have some questions afterwards, feel free to comment on it here.
I think a good one would also be using SCSS Variables.
If e.g. you have a color that you want to reuse you can just create a variable in your file → ”$variablename: #ffee22” and use that in e.g “background-color: $variablename”.
If you want to read more on custom styling you can always go to the docs: (https://docs.mendix.com/howto/front-end/customize-styling-new). I imagine you may have been there already though.