So the styles you want to change are all the responsibility of the browser, and you’ll notice each browser has its own style. This stack overflow covers how to remove that from the HTML Select element:
But to be honest they are a right pain in the backside to begin with and in addition you have the styles being applied using an ID by the WP Forms and UABB Form Styler plugin, so you would need to target those specific IDs which are:
-webkit-appearance: none !important;
-moz-appearance: none !important;
appearance: none !important;
/* add a background image for replacement drop down */
It works in the dev tools but as said they can be a pain so fingers crossed …..