From the Nimbl Portal, under the STORE tab, click on MY SHOP to open the drop down and click on the TEMPLATES button.
HERE you can see what template is currently ACTIVE.
To help make the decision easier, you can click on the PREVIEW button to see what YOUR site will look like using the chosen template.
Click on the PREVIEW button of the template you wish to view your site in.
Choose the color scheme that you prefer
Click on the PREVIEW TEMPLATE button.
A pop-up will appear with a short summary of the template including it's features, how many banners it includes, etc.
Choose your COLOR SCHEME to fit with your brand and click on the UPDATE TEMPLATE button next to the color scheme you would prefer. (You can always change this again, SEE: COLOR SCHEME).