Boosts are originally Mastodon's version of Twitter's retweets. By boosting a post, you share it with your followers.
Behind the scenes, this is how they're implemented on kbin too right now. Though it seems incomplete, as there's not currently a way to view content boosted by your followed users without visiting their profiles manually.
kbin currently uses boosts to sort threads by top, rather than upvotes. Which might be what you were referring to there.
Writing this out now, I realize it might not actually be a difference worth mentioning while it doesn't add anything unique besides added complexity and Mastodon integration (which just goes back to microblogging being a thing on kbin which is already covered).
CSS
CSS stands for Cascading Style Sheets and is a language to describe the visual design of websites.
An older version of Reddit (old.reddit.com) allowed subreddits to specify their own custom CSS code which would be used when users visited those subreddits. They could completely transform the design of the site using that feature. Or they could hack in features specific to their subreddit. Many subreddits both big and small made use of that feature and are still using it even today.
Spoilers for example were done using links and custom CSS long before Reddit added their official spoilers.
Kbin takes the same approach as old.reddit. Magazines can specify their own custom CSS code to change kbin's appearance while visiting the magazine. Though I have only seen one magazine make use of that so far, so it's not nearly as widespread as on Reddit yet.
new comments
On kbin, there's a setting in the sidebar (the gear icon) to mark new comments in threads you've seen before (since turning on the feature). I think it defaults to off.
When turned on and visiting a thread, new comments since your last visit are marked with a yellow bottom left corner. This is a very recent addition and seems to only be clear enough with the Tokyo Night theme currently. On other themes the colored corner is very hard to see for me.
ernest