[JENKINS-70099] Make expand button same style with the input box

[1weiho]

See JENKINS-70099.

Before:

After:

Testing done

Proposed changelog entries

• Make expend button same style with the input box.

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
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Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
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[timja]

just checking did you try this without the importants?

[1weiho]

Yes, it will not work without importants.

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!