It's never been possible for moderators to set sub-specific CSS through the redesign, outside of sidebar widgets. The official line is that this capability will be added after everything else has been implemented. Given how much other functionality is missing, this looks to be a long way down the track. Users will still have the option of switching back to the original design, where your old CSS will still function. You won't have any way to choose which version of the sub your users see. More on reddit.com

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In general, CSS is something in creating websites that allows you to make changes to a website much easier and faster.

Instead of editing every subpage of a website you can edit just 1 file that is referenced to in all of the subpages.

CSS also allows for much more advanced tricks and creating unique website designs.

In the case of Reddit, it allows each subreddit to edit its style by editing its own CSS file. It gives freedom of changes to the moderators of a sub without the risk that they'll be able to edit the website itself.

Ever seen how some subreddits allow you to get a "flair" - an icon (possibly with the addition of text when you hover over it) next to your username? Something like what's seen on r/Steam or r/Undertale (just giving examples from the subs that I use, a lot of other ones like r/todayilearned use that as well).

Gaming/movies/books subreddits can create a spoiler tag for comments - parts of text in comments that are only visible when you hover over it, to prevent people from spoiling things for others.

Subs can have a custom background, or an animation for when you up/downvote. r/Undertale had an easteregg for a long time - a small white dog (related to the game) in the lower right corner.

All of these things are done through editing the CSS by the moderators.

Admistrators now want to redesign the website - to make it, for example, better on mobile. But - there's a side effect. They're planning on removing CSS entirely.

Now - they're saying that they're hoping to introduce parts of the functionality of what subs did with CSS by editing the website itself and adding more tools/options for moderators. But the level of customization that is possible with CSS won't be delivered, because CSS is done by moderators and here admins will have to add options themselves.

If you give people the right tools, they get created and do stuff that you wouldn't imagine possible. The new design will limit the possibilities.

There is, however, one thing that would be pro-change: editing the CSS is often difficult and time-consuming. It's possible to make wonderful things with it, but a new system would be faster for adding some of the currently-done-through-CSS changes. But it would surely make others impossible.

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