Multiple default themes

### Current Behavior

A non-urgent issue, that can easily be worked around by deleting themes.

Currently my themes tabsheet looks like this:


Not sure how I ended up with some many default themes, from which most even have the same color.
Not sure whether these are perhaps there already quite some time from an older dashboard D2 version, or that I have imported them somehow.

### Expected Behavior

Well I am not quite sure which behaviour I would expect, because I really don't know how these themes arrived in my dashboard. But having multiple themes with the same name and colors is somehow strange. Although perhaps not...

### Steps To Reproduce

Don't know unfortunately...

### Environment

- Dashboard version: 1.16.0
- Node-RED version: 3.1.3
- Node.js version: 18.19.0
- npm version:
- Platform/OS: raspberry (Linux 6.1.0-rpi7-rpi-v8 arm64 LE)
- Browser: Chrome

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

I can not provide an initial effort estimate