chore(e2e): switch to Sass theme instead of prebuilt

[mmalerba]

No description provided.

[devversion]

LGTM

[devversion]

Out of curiosity: Why add this here?

We should only add deps that are actively used right?

[mmalerba]

whoops, split this out from a different PR and forgot to remove that

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "branch-manager" is failing
     status "ci/circleci: api_golden_checks" is pending
     status "ci/circleci: bazel_build_test" is pending
     status "ci/circleci: build_release_packages" is pending
     status "ci/circleci: e2e_tests" is pending
     status "ci/circleci: lint" is pending
     status "ci/circleci: prerender_build" is pending
     status "ci/circleci: tests_browserstack" is pending
     status "ci/circleci: tests_local_browsers" is pending
     status "ci/circleci: tests_saucelabs" is pending

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[jelbourn]

LGTM

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