bbPress.org

Here try some of this custom CSS, put it in your child theme or in a custom css plugin.

This is for the submit button

#bbpress-forums #bbp-search-form input[type=submit], 
#bbpress-forums #bbp-search-form input[type=submit] {
    text-indent: 0;
    background: none;
    background-color: #3F51B5;
    color: #fff;
    width: auto;
    height: auto;
    border-radius: 0;
    margin-top: 0;
}

This is for the search input.

#bbpress-forums #bbp-search-form input[type=text], 
#bbpress-forums #bbp-search-form input[type=text] {
    border: 1px solid #3F51B5;
}