BUG: Theme builder error

### Checks

- [ ] I have checked that this issue has not already been reported.

- [x] I am using the latest version of Flow Launcher.

- [ ] I am using the prerelease version of Flow Launcher.

### Problem Description

I tried creating a theme using the Theme Builder on your website; I saved it in the correct folder and restarted Flow Launcher, but when I selected the theme, an error message appeared indicating that loading failed, so I cannot use it.

### To Reproduce

1. ...
2. ...
3. ...

### Screenshots

### Flow Launcher Version

2.1.3

### Windows Build Number

10.0.26200.8875

### Error Log

```shell

Replace this line with the important log contents.

```

Start by reproducing theme creation in the Theme Builder, saving the result in the Flow Launcher themes folder, and selecting it in Flow Launcher 2.1.3 on the reported Windows build. Capture the actual error log and compare the generated theme with the loader's expectations; done means the saved theme loads without an error.

Written by the indexing model from the issue text.