Support Local Filepath for Background Image

[Bejasc]

I work across several machines where the filepath is not always the same.

It would be great if I could include an image to use as my background, by referencing a file that is stored in the repo itself, where the path to the repo does not matter.

e.g config saved as @/.vscode/theme/image.png would resolve to C:/path/to/repo/.vscode/theme/image.png