It's likely a small difference, but the main advantage of having it in the HTML is that it's immediately available, so you don't have to deal with the flash of unstyled content being in the wrong color scheme.

Source: https://css-tricks.com/almanac/properties/c/color-scheme/#theres-a-corresponding-meta-tag-too

You could instead have your "primary" remote (a la "origin" in GitHub) in a VPS hosted by a hosting service you trust. The idea is basically just that you don't actually need a forge to host your git repo, you only the git repo in a file system and an http connection.

If you're actually after decentralized, forgejo (the forge that codeberg hosts) is probably what you want. Either that or, if you don't actually need all the features of GitHub (Pull Requests, Continuous Integration, Issues, etc. via a web interface), you can just keep your git repo(s) in multiple remotes, you don't need a dedicated forge.