With all respect to the topic, the wekeast point of NodeBB is its unfriendly monetization options for webmasters, which keeps several people away from it. Missing app support is also something to mention, but for most webmasters money goes first.
Will offer my way of fixing.. probably something went wrong in the themes, aside from css, probably version mismatches with the core nodebb version your using. The below will revert to working vanilla and lavender theme versions that match w/ your core nodebb install version. (probably need to stick with whatever theme versions it reinstalls, because I've always had problems w/ updating themes, I don't upgrade them anymore) Stick with what came with the nodebb install originally. This will revert back to originals:
./nodebb stop (close nodebb)
npm uninstall nodebb-theme-lavender
npm uninstall nodebb-theme-vanilla
npm install nodebb-theme-vanilla
npm install nodebb-theme-lavender
./nodebb start. try avoid editing too much .less and .tpl in the theme files and stick as much as possible in the Custom css area in your ACP
Every so often we update our installation guide. This time we had a lot to update with MongoDB and CentOS components having big updates since the last guide.
How To Install NodeBB 1.13, NodeJS 13, MongoDB 4.2 on CentOS 8
This is assuming a CentOS 8 Stream Minimal starting point, with "yum upgrade -y" having been run ahead of time. One script to take you from vanilla minimal install to fully running NodeBB community.