Adapt focus-visible styles for checkboxes on images

[JuliaKirschenheuter]

Fixes #2160

Follow-up for #2182

Before:

Contrast difference between focused and unfocused state is under 3:1:

After:

Dark theme:

Light theme:

[emoral435]

LGTM! just squash the commits together and its good :)

[nimishavijay]

Looks good in the screenshots! :)

[JuliaKirschenheuter]

/backport to stable28

[JuliaKirschenheuter]

/compile amend /

[backportbot-nextcloud]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b fix/foo-stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable28

Error: Unknown error

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport

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