Align button design of csv downloader

[keisuke-umezawa]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

Fixes #770

What does this implement/fix? Explain your changes.

The new UI becomes the following

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1497505) 69.52% compared to head (33d913d) 69.39%.
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #780      +/-   ##
==========================================
- Coverage   69.52%   69.39%   -0.13%     
==========================================
  Files          35       35              
  Lines        2359     2369      +10     
==========================================
+ Hits         1640     1644       +4     
- Misses        719      725       +6     

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[c-bata]

@eukaryo @HideakiImamura Could you review this PR?

[HideakiImamura]

I confirmed that the generated download button is appropriate. LGTM.

[c-bata]

Let me merge this PR since already received one approval.