Thank you so much for your email and using our forum :)
I'm afraid only the site owner or administrator would have the ability to modify the theme via the visual theme roller provided with InstantForum. As an end user you can only swap between the themes made available by the site or forum administrator. We do have both light and dark themes within our theme browser for InstantForum but I'm afraid it would be up to reallusion to download the light theme and activate this o you can select this via your user control panel.
Would you mind if I forward your questions to the contacts for reallusion that we have on file?
We'll happily work with them to assist with this but we would need there cooperation.
I look forward to hearing from you and hopefully assisting further,
Kindest Regards,
Ryan Healey