You can't really. Check this snippet from mdn: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select
The <select> element is notoriously difficult to style productively with CSS. You can affect certain aspects like any element — for example, manipulating the box model, the displayed font, etc., and you can use the appearance property to remove the default system appearance.
However, these properties don't produce a consistent result across browsers, and it is hard to do things like line different types of form element up with one another in a column. The <select> element's internal structure is complex, and hard to control. If you want to get full control, you should consider using a library with good facilities for styling form widgets, or try rolling your own dropdown menu using non-semantic elements, JavaScript, and WAI-ARIA to provide semantics.
Do indeed check the advanced guide for the little you can do to style them. A good example of how bad styling is for options:
You'll notice that the options don't inherit the font set on the parent. You also can't consistently set things like spacing and colors. For example, Firefox will apply color and background-color when set on the <option> elements, Chrome won't. Neither of them will apply any kind of spacing
<select>not <'option>` I was looking how to styleoptionso farselectonly can be styled in CSS