Switch to `.btn-secondary` for the search button so that it's accessible

Created by: kadimi

With the btn-outline-success class, the color margin is not enough as opposed to btn-secondary.

See Contrast Checker results for both

• https://contrastchecker.com/?c=28a745&b=212529 (fail)
• https://contrastchecker.com/?c=ffffff&b=6c757d (pass)