feat(sumac): add css support

[Asespinel]

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/learning
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@$paragonVersion/dist/core.min.css"
    },
    "defaults": {
        "light": "light"
    },
    "variants": {
        "light": {
            "url": "https://cdn.jsdelivr.net/npm/@openedx/paragon@$paragonVersion/dist/light.min.css"
        }
    }
}

[dcoa]

@Asespinel the test are not passing due to a linter issue, could you have a look on it? (in your local environment you can run npm run lint)

Looks like is a file you are not changing my sugest is unable the linter rule in that line.

[jignaciopm]

@dcoa Lint error issues were fixed. Currently, the errors presented in the PR are associated with the unit tests, to which we will not dedicate time to be able to close this PR.

[bra-i-am]

LGTM... I would only call for publishing the new package versions in NPM, but it's not a blocking request