You can use custom stylesheets with team sites in SharePoint
They can even inherit CSS and master page settings, despite the master page link not being present in the site settings – it is still accessible via direct URL as long as your site collection has Publishing Infrastructure enabled.
To access the master page (and CSS) settings on a team site, add this to the end of your site URL:
Once here, you can set your master page, tell your team site to inherit your theme, inherit your CSS, or attach a different stylesheet.
Bake inheritance into your site template
If you find it super annoying to go to this URL and set all these things for every site you create, simply set them on a bare-bones template site, then save the site as a template. Sites created from the template will keep the inheritance settings. You can also push down theme/css/master page settings from the root site in your site collection using the checkboxes in the master page settings.
Looking for a starting point for CSS?
There’s a few lines to get you started here.