Hi,
The colors of the background and the tick can be changend under the Theme Options -> Settings icon & dropdown panel: https://i.imgur.com/APOEdVd.png
The size is a bit more problematic, as it is an absolutely positioned element, and changing the size, you might have to change it’s position as well. This custom CSS rule will allow changing all attributes for that, it includes the default size:
[html].asp_option_inner label {
width: 17px !important;
height: 17px !important;
top: 0px !important;
left: 0px !important;
}[/html]
Changing the width/height, you will most likely have to change the top value as well. You can use negative pixel values for the top/left, in case you need to position it upwards.