[dashboard]CSS Templates not working

[syazshafei]

A clear and concise description of what the bug is.

Expected results

Black background is preserved after refresh

Actual results

Black background missing after refresh

Screenshots

How to reproduce the bug

1. Go to 'Any dashboard'
2. Click on 'Edit dashboard > Edit CSS'
3. Save CSS template
4. Refresh browser
5. See error

Environment

(please complete the following information):

• superset version: 0.38.0
• python version: based on superset version 0.38.0
• node.js version: based on superset version 0.38.0

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.

[junlincc]

could it cause by our native filter feature flag? @rusackas