You need Custom CSS Loader to apply this extension, like Vibrancy.

Usage

1. Press F1 then type "settings json" and press ENTER.
2. Edit and paste following code:

  "vscode_vibrancy.imports": [
    "<HOME_DIRECTORY>/.vscode/extensions/eyhn.vscode-vibrancy-1.0.8/themes/Dark (Only Subbar).css", // if you are using vscode-vibrancy, this line is needed for applying translucent effects. Change "Dark (Only Subbar).css" to your favorite theme.
    "file:///<PATH TO CLONED REPOSITORY>/all.css"
  ]

3. Press F1 then type "Reload Vibrancy" and press ENTER.