[Grace Themes]

Hi

Go to the Appearance -> Editor -> style.css and add below css code in style.css file

OR

Go to the Appearance -> Customize -> Additional CSS
and use below css code

.search-field { font-size: 14px; height: 30px; padding: 5px; }
.search-submit { background-color: #5d5a5a; border-color: #ccc; border-radius: 0; border-style: solid; border-width: 0; color: #ffffff; padding: 7px 10px; }

Regards
Grace Themes