User-CSS

especially in the German blocking thread it came out how users differ in the block content being shown (or not) and in the end the way of how it's hidden will be decided will by by the admin and/or the theme used, which may not satisfy all users which is a perfect time to put this Idea forward.

User-side CSS/SCCS/LESS/SASS/whatever

in the end I dont care what kind of CSS pre-language will be used (or any at all) the only thing I would want would be that I could style without having to rely on tools like stylish (which may not even be available everywhere).
in the end it gives the user a way to make WCF a lot more likeable in many ways without needing the admin to create themes which may affect anyone and needs unneeded effort at the administration while it kills literally nobody as this is already partially available using stylish and other browser-side tools.

in short in the german "enhance blocking" thread over at WBB enhancements forums WBB staff said that they want to make it not completely hidden, but instead expandable, other users disagree and want to include a display:none for that in their forum which I merely used as an example to how user's opinions about certain stuff are different., in the end if the admin says hidden the only way for the users to maybe see something (maybe because it might be important to the discussion) he wont be able to see it without tools like stylish or similar things, which is particularly annoying if you are on a platform that doesnt support that.

This is just one example.
this can also be used that people who have bad eyesight to improve contrast and so on.
or just making fonts larger, or if they have problems (for whatever reason) with a color then they could change it, there are so many uses...

also even though it manipulates the website's design it is just doing this for himself, the main difference from stylish etc being that these are applied everywhere and not just locally.

I dont exactly know why you have a problem that certain users can enhance (from their own perspective) the site for themselves, especially since there are ways to do it in the browser (and actually I saw some sites who actually have this, not many but I did see some)

it isnt as if the users gets to acp and change the style for everyone.

I did say that there are tools like stylish etc (which is essentially what userstyles was made for) but they
1) usually arent on all platforms e.g. phones
2) dont sync over different browsers/PCs/etc
3) you may not be always able to install such addons.