[Dashboard] Color scheme won't be discarded when discarding changes

[geido]

A clear and concise description of what the bug is.

How to reproduce the bug

1. Go to a Dashboard
2. Click on Edit dashboard properties
3. Change the color scheme and click Save
4. Without saving the whole dashboard, click on "Discard changes"
5. The page will refresh and the color scheme change hasn't been discarded

Expected results

Saving changes in the "Edit dashboard properties" Modal should only be effective when the whole Dashboard is saved. When discarding the changes, any change should reset.

Actual results

The color scheme is not discarded

Screenshots

untitled.dashb.2.mp4

Environment

• browser type and version: Chrome
• superset version: master

[geido]

@junlincc @jinghua-qa FYI