When updating the CSS in Portal from dashboard/site-settings/appearance, a large number of line-breaks (not necessarily total character count or file size) can cause the API Portal to break and error out. You will be presented with the following error upon hitting the save button:
<Please see attached file for image>
Release: L7APIP99000-3.1-API Developer Portal-Perpetual
The line break limitation is looking to be addressed and fix in the upcoming CR of Portal. But for now, at the dashboard/site-settings/appearance you can import the css from a reference under resources folder. Example below:
Importing and referencing it this way will not cause the Portal to error out, even with the same amount of line breaks.