This PR fixes MPP-3821.

Bug description

In light pages like /premium, the sign up button was invisible.

Screenshot of fix (if applicable)

[x] Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
[x] Customer Experience team has seen or waived a demo of functionality.
[x] All acceptance criteria are met.
[x] Jira ticket has been updated (if needed) to match changes made during the development process.
[x] I've added or updated relevant docs in the docs/ directory
[x] Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.
[x] All UI revisions follow the coding standards, and use Protocol tokens where applicable (see /frontend/src/styles/tokens.scss).
[x] Commits in this PR are minimal and have descriptive commit messages.
[x] l10n changes have been submitted to the l10n repository, if any.