i can suggest checking in browser F12 console (network tab) for errors related to CSS files, then check your http server logs for errors (also related to CSS files) and go from there.
do all that in a clean browser profile, without addons, or an incognito window to exclude addons like adblock blocking parts of tt-rss by mistake.
i have no idea what “Caddy” even is so i can’t really help you with it specifically.
also, try doing view source on tt-rss index page and post
<link> elements related to CSS, i.e.
<link rel="stylesheet" id="theme_css" type="text/css" href="css/default.css?1553831585"/>
login page should also reference
css/default.css which you should be able to open in browser, btw.