It took me a little while to figure it out, But I think you can do that by doing the following
Add the rule below in Main Options > Fonts & Custom > Custom CSS Rule box
#header-search .search-field,.header-search .search-field,.menu-search .search-field {background-color:#f0f0f0;border:2px solid #c3c0ab;cursor:text;outline:0;width:230px}
If you are changing any of the values in that rule, you will need to also add the following one with the same modified values
#header-search .search-field:focus,.header-search .search-field:focus,.menu-search .search-field:focus {background-color:#f0f0f0;border:2px solid #c3c0ab;cursor:text;outline:0;width:230px}