guide for getting started creating styles, and hover box styles

Is there a guide or tutorial to creating a new style for beginners?
Is it possible to fork existing styles?

Last and this is most specific, does anybody know how to modify style of hovered over element on Wikipedia? I want that element to be purely opaque so it forms a strong background, not translucent. I don't care if it goes back to original white or stays the modified color as long as you can clearly read the text.

I'm at week one of a web dev bootcamp btw so my skills are modest but they are going to grow! :)
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