Fix theming: Theme specific variables must be set on the root node

[susnux]

• Resolves: Black table on light theme text#3369

Summary

Currently if you use a css variable which links to an other css variable, like --color-main-background and a theme is enabled, the theme is not used for the variable. See linked issue (same e.g. on forms, see nextcloud/forms#1471 (review)).

The problem is the css variable evaluation order, e.g. consider the browser requests dark color theme but the user selected the bright nextcloud theme, then:

• HTML tag
  • defines --color-main-background for dark color theme (as requested by the browser)
  • defines --table-color-background to be the same as --color-main-background
• Body tag
  • redefines --color-main-background to match the theme (because data-theme-light is set on the body tag)
• Table tag
  • Uses --table-color-background which evaluates to HTML-Tag::--color-main-background and in the context it is set to dark instead of bright.

So the solution is to set data-theme-light on the HTML tag rather than the body tag.

Screenshots

System uses dark color theme, but nextcloud is configured to use the bright theme.
You can see the NcSelect has the wrong background color:

before (currently)	after (with this PR)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

Superseded by #36462