Colors and presentation are defined in

yoursite.com/ow_plugins/memberads/static/themes/default/memberads.css

In line 90 the element item_list_table is defined and in line 98 item_list_table_isPaid. (blue color in the image)

One or the other is applied depending on whether the member is paid or not (user roles)

In red you will see that "border" is defined, one does not have (none) and the other indicates the thickness (5px), if it is continuous or dotted, and the color.

You can also define the radius of the curve (border-radius) and the background color (background-color)

In addition to the basic colors (white, blue, red, green, black ...) you can define your own color, in this case instead of a name its value is indicated in hexadecimal with the # sign in front, as you can see in the background-color in paid members.

You can find out the code for all colors with this free selector:

https://www.w3schools.com/colors/colors_picker.asp

I recommend that you register (it's free) at https://owspain.com because I will not publish more plugins or more updates here due to the abusive conditions of this store.