Semi-related, my big hack in my instance's custom CSS was to substantially enlarge the Show More/Show Less buttons to make it a much easier target to tap/click, which makes it much easier to expand or recollapse a post.
This sounds handy. Out of curiosity, does Mastodon have any form of extension/plugin system yet, to allow you to manage these through upgrades and/or distribute them easily, or are you modifying the stylesheets directly?
So far I've been sticking to things I can do purely in CSS (I wanted to move the CW subject lines into the buttons, for instance, but couldn't find a way with the current markup). For that, the Administrator section of Mastodon does keep and store a custom CSS section that it will output as a <style> block on page renders. I've been sharing my CSS changes on my account as I make them, and other admins could just copy and paste them.
(Probably the most major other change was I fought the CSS to make the background 100% black for OLED power savings.)
I'm told there also is a deeper theming system and some shared themes as Ruby gems. I haven't explored that personally because I'm using a shared host and so far I've been happy with the simple custom CSS overrides.
