Body css overriden

Now i have set up a plain kirby.
Now i have somewhere
body { margin:0 …
that overrides the
body {margin: o auto …

What is the best practice to correct this?
Use important!
Delete body {margin:0
Have a file for custom css, that is loaded last and overrides all css from before?

Seems i play on myself while firefox doesnt reload the css. (Cache is off);
so i have to test changes in Chrome.

I don’t quite understand. If you are talking about the Plainkit, it doesn’t come with any styles, so you add a style sheet yourself.

Try to avoid !important.

Firefox is a really ***, you change things on a web, but nothing changes.
Then you try another browser and everything is ok.
Normally the last definition overwrites the rest, normally you load custom.css as the last one. I do this and everything is fine now.

That sounds as if you are using a CSS framework like Bootstrap. Would be helpful if you provide such information in your post.

As regards Firefox, with dev tools open and caching disabled in dev tools, it should reflect any changes (after reloading). Or reload with Cmd+Shift+R (on a Mac).