CSS Formatting for Drop Down "Selected" Custom Field

Hi,

Well, that's not really linked to HikaShop.
You need to add either custom CSS or custom CSS and custom javascript to the page where the dropdown is being displayed. It depends what you want to do exactly.
Here are some examples:
stackoverflow.com/questions/17698110/how...ion-in-select-option
You only have to change the class/id used in these examples to the class/id of the custom field's dropdown in order to get the CSS/javascript to work on the custom field.
You can easily find the class/id of the selects with the inspector of your browser:
kb.oboxthemes.com/articles/how-to-get-css-styles-for-elements/
Regarding where to add the CSS, you can do it in the HikaShop CSS:
www.hikashop.com/support/documentation/1...ize-the-display.html
And for the javascript, it depends what code you want to use and how you configured your custom field so I can't say much more on that for now as I don't know either.