fix: ensure enabled themes are set on the template

[susnux]

Summary

Ensure enabled themes are always set on the template.
isEnabledForUser is not needed since Nextcloud 22 as this app is force-enabled for all users.
Also class_exists is not needed as this is shipped since 22. Moreover the check prevented the fallback to enforced theme when no user is logged in.

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]

/backport to stable30

[susnux]

/backport to stable29

[nickvergessen]

Fixes the problem

[backportbot]

The backport to stable29 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable29
git pull origin stable29

# Create the new backport branch
git checkout -b backport/51929/stable29

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 00580b36

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/51929/stable29

Error: Failed to check for changes with origin/stable29: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot]

The backport to stable30 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable30
git pull origin stable30

# Create the new backport branch
git checkout -b backport/51929/stable30

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 00580b36

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/51929/stable30

Error: Failed to check for changes with origin/stable30: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.