Hi 3SpottedZebras,
In order to change the text color of your seller details profile to white, you will first need to click the "Manage Seller Details" link under the "Profiles" tab within your account and select the "Edit" option next to the desired seller details profile. You would then need to highlight the text in each section with your cursor and select "white" from the "Text color" drop-down menu.
I don't believe it is possible to modify a template to make the seller details sections reflect white text automatically so you may want to create a separate seller details profile with white text. You could then choose that one when using a template with a dark background and the other one when using a template with a light background.
If you have any additional questions or concerns regarding your Auctiva account, please feel free to contact our customer support team by completing the web form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx-Mike