Small CSS rant + Asking for advice

Making a site JavaScript-less with bootstrap but the CSS is kicking my ass- I do the code directly as it is meant to be, then I try to add one thing and it breaks. I'm gripping on w3schools for dear life and I just can't seem to wrap my head around anything other than the basics. CSS is pain ESPECIALLY when I'm doing it on an external sheet. (I don't want to do internal because all the text gets overwhelming.) Anyone have some ideas to help with this?

Edit: So I realized the browser tool thing is really easy for visuals + that BOOTSTRAP IS INSANELY VAST. For just about every CSS element theirs another 1.5k sub rules which is great for getting specific but not great when you are basically creating a rule for a already ruled element that you have no way of finding easily. Bootstrap is just a functionality CSS sheet I think and not the equivalent to a HTML DLC

(Image is my CSS sheet compared to the crazy amount of CSS sub sheets that exist in bootstrap. My measly little 16 rules look pathetic)