Worked it out.
In the admin panel:
then tick the 'upload files' box for registered users.
I want to edit my theme and customize it, but in order to do so you have to restart the forum every time you make a change, which makes it extremely hard to visualize and work on it.
If you just add custom css on top through the admin panel, it automatically saves and updates the page in a few seconds. But with a custom theme it takes like 30 seconds at least for every change, which is terrible for development.
Please tell me there's some method I can use to visualize my theme changes faster.
We recommend you use
front grunt which will rebuild only the assets that changed and restart automatically.
You can also use your browser developer tools to apply new CSS styles or modify existing ones, though you'll still need to persist those changes into code manually.
@pitaj did you mean "grunt"?
Yes he meant grunt