feat: add css variables support sumac

[jignaciopm]

Description

This PR intends to add CSS Variables support to th Sumac release.

How to Test

1. Create a Sumac environment you can use Tutor or TVM.
2. Mount this MFE using this branch using tutor mounts add path/to/learner-dashboard
3. Run tutor config save
4. Add this config to you development.py file or to your tenant settings:

MFE_CONFIG["PARAGON_THEME_URLS"] = {
    "core": {
        "url": "https://cdn.jsdelivr.net/npm/@openedx/paragon@23.0.0-alpha.7/dist/core.min.css"
    },
    "defaults": {
        "light": "light"
    },
    "variants": {
        "light": {
            "url": "https://css-varsify.s3.amazonaws.com/public/6d17ef6a-a0e0-41ce-a08e-01b6fffa1b17.css"
        }
    }
}

[Asespinel]

It looks good to me.