Dialog groups do not use chosen theme background

### Current Behavior

When the default theme is changed, or another theme is applied, dialog groups do not have the `!important` flag in their CSS to use the configured background color.

group background-color uses `!important` flag.

Dialog background-color does not use `!important` flag.

Setting the `!important` in the browser inspect pane displays the correct dialog group background color.

### Expected Behavior

Dialog groups should use the configured theme colors just as all other groups do.

### Steps To Reproduce

Set up a dark theme with a dark group background and deploy.

### Environment

- Dashboard version: 1.22.0
- Node-RED version: 4.0.7
- Node.js version: 20.18.1
- npm version: 10.8.2
- Platform/OS: Ubuntu 24.04.1 LTS
- Browser: Chrome

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member