release-23.1: ui: update crl-email-subscription styling

[blathers-crl]

Backport 1/1 commits from #114538 on behalf of @sudomateo.

/cc @cockroachdb/release

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Previously, when the background image failed to load for the
crl-email-subscription CSS class it would render render white text on
a white background, making it difficult to read the call to action.

This patch adds the background-color property to the
crl-email-subscription CSS class so that it is used when the
background image fails to load.

Fixes: #105740
Part of: CRDB-29170

Release note (ui change): The overview page now correctly renders the
background color for the email signup, fixing an issue where it was
difficult to read the text.

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Release justification: Fixed a UI issue preventing users from reading the email subscription call-to-action.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[dhartunian]

LGTM. Thanks!

[joshimhoff]

@dhartunian @maryliag, does this PR fix a "serious issue" as per the policy (https://cockroachlabs.atlassian.net/wiki/spaces/ENG/pages/1043235321/Database+Backport+Patch+Policy#Getting-your-backport-reviewed)?

Backports should only be created for serious
issues or test-only changes.

[dhartunian]

@joshimhoff I'd say in terms of the size of diff to impact ratio, making the first page of the DB Console not look terrible is "serious" enough to merge this.

[maryliag]

Agree with David here, not being able to read a text it serious enough.

[joshimhoff]

Thanks!