@Panda if it’s hosted NodeBB then I’m 99% sure that they will be using nginx. Nginx itself will cache static assets - css being one of them, so what you are experiencing is normal behavior, and the cache-control
headers you have defined with have virtually zero effect when they are already set by nginx itself.
On a desktop, you can force the browser to clear that cache for that specific site by pressing f12 to open the developers console, then left click and hold on the reload button in the browser
After and one second you’ll see three options. You want the third, which is “Clear cache and reload”
I know you use your phone to do most things, but I can tell you now that it’s almost impossible to develop in this way and it needs to be done via a PC or laptop.