Hi and welcome to Plone!
You can use conditions in your theme if you know the paths: https://docs.plone.org/external/plone.app.theming/docs/index.html#available-rules
<theme href="theme.html" /> <!-- Default theme -->
<theme href="news.html" if-path="/news" /> <!-- Custom theme for /news -->
<theme href="frontpage.html" css:if-content="body.portaltype-plone-site, body.section-front-page" /> <!-- Custom theme for Plone homepage -->
The last rule applies to all content with type
plone-site (which usually is only the root of your website) and a document (section) with id
front-page (which is the default Homepage document). If you use another one as your homepage, e.g.
homepage, it would be
body.section-homepage. But you can see it from the generated HTML markup. Also, if you have a custom template in Plone applied, you could check on that too:
<theme href="frontpage.html" css:if-content="body.template-mytemplate" /> <!-- Custom theme for a template. -->