Yes, that’s not part of the theme. It is part of WordPress’ default widgets. You can still change it though by adding this into a child-theme functions.php
function change_search_placeholder($text) {
return str_replace('placeholder="Search …"', 'placeholder="Search Text"', $text);
}
add_filter('get_search_form', 'change_search_placeholder');
function change_search_submit($text) {
return str_replace('value="Search"', 'value="Search Now"', $text);
}
add_filter('get_search_form', 'change_search_submit');
You can also leave the replacement texts blank (ex. value=””) if you don’t want them to show.