At least they did not ask you for screenshots on a platform that does not allow attaching them.
As for your question, is there a reason you do not want to use inline css or override css?
Anyway, you would define the CCS override for: WCJ_General_Shortcodes.wcj_currency_select_drop_down_list
Specifically that shortcode exists in : class-wcj-widget-multicurrency.php If you want to view it’s structure.
Booster-Plus-For-Woocommerce/includes/widgets
Which suggests that you could also override WCJ_Widget_Multicurrency
Of course you also have the high maintenance option, but overriding the shortcode style is the most durable option across updates.