I've installed the "Solid" theme, it just gives me a blank home page and nothing is visible, what did I do wrong?
Currently on PHP Version 7.2.31
Your site is now using bootstrap4 theme. So no idea.
My guess - wrong version of core e107.
It should look like this:
Github version 2.3.0
2.3.0 themes will not work with older core version. NEVER, they have features that weren't supported before.
Maybe somebody has 2.1 version, but I would sync to latest github version.