Here is documentation on starting your own theme: https://docs.nodebb.org/development/themes/
If you want to make small style changes, it's best to do it in the custom CSS portion of the ACP, instead of editing the theme files directly.
I installed nodebb and it works great with the persona theme, when I try installing it with other themes (even default ones), most or all of the css just does not load.
I tried looking for the css file with ctrl+u, and for lavender
<link rel="stylesheet" type="text/css" href="/assets/client.css?v=b8eg2pvnjhs" />
takes me to a not found page.
Oddly enough, the client.css shows valid for the oxide theme, but I still dont think its all the css for the theme
For context I am running nodebb v2.2.5 using docker
my routing also uses docker on a seperate container