I almost never bother theming things on my computer; I usually don't really care about how things look and I can't be bothered with it.

I have two exceptions to this: NeoVim and tmux.

tmux in particular looks very ugly out of the box; I'm not sure why they decided to have a bright green bar on the bottom (presumably to be high-contrast in a low color terminal), but I really hate staring at it all day (since I kind of live in tmux most of the day on both my work and personal computer). I have it set as close as I can to the Wombat color scheme that's built into emacs (and which can be easily added to Vim), because I personally have always felt that that was the most pleasant theme to stare at for long periods of time.

Another thing I do is set the prefix key to backtick. I have no idea why I started doing this, but at this point I'm too geriatric to change, and I still greatly prefer this over ctrl-b. The only issue I've had is when I need to do code blocks in markdown, but I've just gotten used to hitting backtick twice when I need to actually use a backtick.