Fix learn gitlab popover

What does this MR do?

There seems to have been a regression that this MR fixes for the learn-gitlab popover. Merge Request that introduced the popover: !33194 (merged) had a blue background and visible close icon, which is no longer the case.

No changelog since this is still behind the feature flag of the onboarding_issues experiment.

Screenshots (strongly suggested)

Before (buggy styling)	After (fixed styling, same as expected)

How to test

By going through the flow

• in the rails console: Feature.enable_percentage_of_time(:onboarding_issues_experiment_percentage, 100)
• Sign up as a new user
• Go through the whole signup flow, create a group and a project. You will end up on the group overview page

By creating a project

• Create a project named learn-gitlab in a group
• Set the cookie onboarding_issues_settings with { "groups#show": true, "projects#show": true, "issues#index": true }
• Refresh on the group overview page and see the first popover

There are multiple popovers:

1st Popover (group overview)	2nd Popover (project overview)	2nd Popover (in project issues page)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team