Since select boxes are rendered by the OS as a drop down list itself, it is often out of the reach of CSS when it comes to styling options. However, there are other ways to highlight options like using an <optgroup> (https://developer.mozilla.org/en-US/docs/Web/HTML/Element/optgroup) which can organize options into groupings if you need to differ between options.
The select itself can also be styled in many ways, just that the options themselves are limited in their styling potential. Sometimes increasing the font size will also increase the space around items and give you that feel of more spacing. So that is also something you can try.
There are several websites showing how to customise a select element (or something that looks and hehaves like a select element).Here for example are 31 methods: