fix: fix flash of light theme with SSR, fixes #17953

[stefanvanherwijnen]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)

Other information:
This simply checks if the body--dark class exists and sets the initial dark mode accordingly. At first glance this seems to work fine, but I am not sure what the earlier problem was exactly (c6368ba).

[github-actions]

UI Tests Results

    1 files     98 suites   39s ⏱️
1 031 tests 1 031 ✅ 0 💤 0 ❌
1 050 runs  1 050 ✅ 0 💤 0 ❌

Results for commit 84ae6e9.

♻️ This comment has been updated with latest results.

[stefanvanherwijnen]

It obviously was not that simple 😅 .

[rstoenescu]

Tweaked it a bit and merged it.
Thanks for contributing!

[rstoenescu]

Just make sure that the problem is NOT actually that you're not setting the same dark mode value for both client and server.