Step 1: Get SharePoint collection path.
Log in as an administrator at your site collection. (VERY IMPORTANT! to mark the "remember me" checkbox before pressing Login button).
Go to site settings -> Design manager -> upload designfiles and copy the linkadress that's in there.
Step2: Map network drive
Go to File explorer on your computer and rightclick on "Network". Choose map network drive... choose a drive and press finnish.
This wil only work if you have ckecked the "remember me" button at step 1
Step 3: Add css to the drive
Add a folder containing your css. (to override exsisting css you have to look at the sites source code (F12 when you are on the site you want to style) from your browser and find and copy the classes or ids and put them in your css file here in the mapped drive folder. Thats how you override them.
Step 4: Upload to Sharepoint
Go in to site settings -> Master pages and page layouts. Find yor css in here and hit the arrow down symbol to the right of the item. hit "Publish a Mayor version" and now it should work.
If you want to do this for the public page you have to do step 1 again but you have to be on the adress yoursite-public.sharepoint.com instead of yoursite.sharepoint.com