As you know, the officiel Theme Palette is quite limited concerning customization of your site.
I've personally been using SharePoint Framework to customize it's appearing.
In your project, you may know that you have SASS customization, which is by default scoped to your components, but nothing prevents you from including a standard global CSS file, or scoping your SASS file to global (
Unfortunately, your customization files will only be loaded after the whole framework and you will be able to see before/after at page load. The silver lining here is that it only appears once in your user navigation since the SharePoint Modern Experience does not reload the whole page as long as your navigating inside your Site.
Don't hesitate to ask for more details,
Hope this helps !