Allow use of custom data-theme values that also override auto light/dark switching

[wmurphyrd]

I'm migrating https://github.com/immers-space/immers over to pico in order to make the style customizable for each server, and I love it. The extensive use of CSS custom properties means we can customize it at runtime without having to rebuild the web client.

We'd like to offer an additional built-in theme that is activated via data-theme attribute value, but we have an issue that setting data-theme to anything other than light leaves the auto light/dark switch enabled, making that particular theme unreadable for prefers-color-scheme: dark users.

This tiny change makes it so any value for data-theme will disable the auto-switching. All of the existing behavior is preserved:

• Users with no preference on page with no data-theme gets light mode
• Users with dark preference on page with no data-theme gets dark mode
• Users with light preference on page with no data-theme gets light mode
• [data-theme=light] shows light mode regardless of preference
• [data-theme=dark] shows dark mode regardless of preference

2022-11-03.11-10-33.mp4

[lucaslarroche]

Hi @wmurphyrd,
I finally took the time to look at the PR and do some testing.
Thank you. You are right, that's how it should be.

I will merge your PR into dev, then I will add a few more fixes before merging into master.