Actually you can only style few CSS properties on :modified option elements.
color
does not work, background-color
either, but you can set a background-image
.
You can couple this with gradients to do the trick.
option:hover,_x000D_
option:focus,_x000D_
option:active,_x000D_
option:checked {_x000D_
background: linear-gradient(#5A2569, #5A2569);_x000D_
}
_x000D_
<select>_x000D_
<option>A</option>_x000D_
<option>B</option>_x000D_
<option>C</option>_x000D_
</select>
_x000D_
Works on gecko/webkit.