[Feature Request]: Grey option for main and secondary color theme

### Guidelines

- [X] I have searched the [issue tracker](https://github.com/FreeTubeApp/FreeTube/issues) for a feature request that matches the one I want to file, without success.
- [X] I have searched the [documentation](https://docs.freetubeapp.io/) for information that matches the description of the feature request I want to file, without success.

### Problem Description

Can you add a Light-Dark Grey color option for Main and Secondary color theme ?

### Proposed Solution

Can you add a Light-Dark Grey color option for Main and Secondary color theme ?

### Alternatives Considered

Can you add a Light-Dark Grey color option for Main and Secondary color theme ?

### Issue Labels

new feature, new optional setting

### Additional Information

Here are some color sample suggestions:

Davy’s Gray
Hex #555555
RGB 85, 85, 85
CMYK 0, 0, 0, 67

Dark Gray
Hex #A9A9A9
RGB 169, 169, 169
CMYK 0, 0, 0, 34

Battleship Gray
Hex #848482
RGB 132, 132, 130
CMYK 0, 0, 2, 48

Gray
Hex #808080
RGB 128, 128, 128
CMYK 0, 0, 0, 50

Dim Gray
Hex #696969
RGB 105, 105, 105
CMYK 0, 0, 0, 59