So I actually got it working. CSS is now back.
Although, how do I go about making the CSS work with HTTPS?
Hmm, I can't load your forum in a first attempt because of your self signed certificate (a dummy certificate of your webserver), even I request your site with HTTP, not HTTPS. When I omit the warning and load your forum with HTTP, I get a formatted page with the default theme. So far, so good.
http://) or HTTPS (as
In your case the link to the CSS file is
http://forum.reziem.online/themes/default/style.min.css so as it is written here.
The URL for the ressources must be accessible. If you set the forum URL with the protocol HTTPS but your forum is not accessible via HTTPS but only with HTTP, also the CSS ressources are not loadable. Your forum isn't accessible via HTTPS so the design was broken because the linked CSS file was not accessible.
Is your installation self hosted or have you a hosting provider account? If your forum should be publicly accessible over HTTPS (this is strongly recommended nowadays), you need a valid and not self signed certificate. Take a look at Let's Encrypt. Your hosting provider may offer Let's Encrypt certificates for you. Here in Germany this is common and mostly for free.
If your certificate is active, you can set the forum URL in the settings to
https://forum.reziem.online (pay attention to the "s" in the protocol) and, if possible, you can enforce the use of HTTPS for requests to your site and forum afterwards.