By default the stylesheet from /r/carbon_beta is applied but you can change that by editing the userscript after installing it.
There are three variables you can change,
subreddit: which is where the stylesheet is pulled from
snoo_logo: which is reddit's logo on the top left, subreddits usually have their own logo so you need to change this as well to make it look best
custom_css: if you're familiar with CSS, you can make your own little changes here
Note that updates will reset any changes you make on the above variables so you might want to disable automatic updates for it (only check by yourself when something breaks). Though I don't plan to update it often.
On Chrome you need to enable two flags: