You can customize your flarum appearance through the Appearance pane in the admin dashboard. Options there include night mode, colored header, and theme colors.
Everything else is up to extensions and custom CSS. There is no "amount" of themes/skins: there are as many as the community (or you) creates.
It is currently not possible for users to choose a theme. This is something we might (no decision yet) support in the future, but if it happens, it would happen sometime after the stable release, as it is not a priority at the moment.
