Board Thread:General Discussion/@comment-36153576-20200510170009/@comment-9605025-20200510222745

What Love Robin said is true. However, if I read your original post correctly, that wasn't the question you were asking.

How CSS is applied is a bit more complex that just which sheet it comes from. It involves something called specificity. The CSS rule with the highest specificity is applied regardless of its source. As described in the article, the only time the source matters is when competing rules have the same specificity. In that case, the rule that is "further down" gets applied. In order for your personal CSS to be considered "further down" than the site CSS, it must be loaded after the site CSS. This is the case for legacy wikis and should be the case for UCP wikis as well. That said, UCP currently has some loading order issues with Fandom's global CSS; so I wouldn't be surprised if personal and site CSS are also loaded in the wrong order.

JS is harder to say as scripts are run asynchronously. In other words, they run at the same time. If two JS scripts attempt to set different values for the same global variable, the final value of that variable depends on whichever script touched it last. This means it could be one value on some occasions and the other value on other occasions. This situation is known as a "race condition".