The ThemeBuilder Branding section allows you to upload your logos for a light and dark version. These are used within the ThemeBuilder Logo Element.
Set the ThemeBuilder Branding
To set this up, head over to "ThemeBuilder" and "Branding" where you will see the options to upload a light version of your logo to be used on top of dark backgrounds, and a dark version to be used on lighter color backgrounds. By default you'll see the OptimizePress logo, so just upload your own by clicking the upload button and choosing your logos.
You can also set the logo link here as well. Don't forget to save the settings also: 
Using the Logo Element
The logo element can be used within the ThemeBuilder anywhere you need to use a logo. Just drop in the element and it will pull in the appropriate version of your logo.
To add the logo element to your ThemeBuilder template part, just add it like you would any other element.
Just make sure to choose the correct version of your logo after adding the logo element to the page.
If you want to change the size of the logo, you can do that in the toolbar like this (below). Just change the max width:
