Hey Thiago,
This appearance is created by Webkit browsers rather than Webflow’s css. You can reset this with css as follows;
In your site’s header (Dashboard > Custom Code)
<style>
select {
appearance: none !important;
-webkit-appearance: none !important;
-moz-appearance: none !important;
}
</style>
This will get rid of the default arrow. You the use the Webflow designer to add a background svg arrow image with the dimensions as needed.