Struggling with CSS <<

Hey! First off, this is my first time coding CSS. I have very little HTML knowledge, and only know how to change the color of things when working on CSS. I'm having a super hard time with coding my own CSS. I've tried going over MULTIPLE guides both on Lioden and Wolvden, but I continuously get lost.

1) The little white margin that have the caves and wolves, and all that. What code do I use to lower the opacity of that? And where does the code go? In body, or ....

2) My background won't change anymore, and I can't figure out how to make every box/border on my den page have a lower opacity so it's sort of see-through. 

3) How do I code borders/boxes, or those boxes that sort of drop down when you hover over them? I saw someone do it, so I tried it as well, but it didn't work, so I'm assuming they coded that into the CSS so it would automatically be in their den page editor. 

Please ignore everything that's messed up in this, I had it all ready and everything was working fine before, but I got frustrated and tried to force code that won't work -- to well -- work, and that didn't... work out. I don't like how many times I said work lmfao. If you see errors, please feel free to help me correct them! You can also DM me instead of replying here -- 

Click me to be taken to doc with CSS 

I've been using this as a guide with my own work, as well as what I learned from asking others, maybe it can help you out!