Choose a Campaign Builder theme to customize your work space, and use the Auto Layout icon to lock/unlock the positioning of your panels.
Click on your profile pic in the top-right corner of your Builder, and then click on either Light Theme or Dark Theme from the drop-down menu, depending on which one is currently in use.
The Dark and Light Themes simply skin the Campaign Builder; there are no differences in features or functions between the two.
By default, Auto Layout is turned on. This places your Campaign Builder panels on the left and right sides of your screen, as shown below - the Add Widgets Panel appears in the top-left, the Edit Widgets Panel in the bottom-left, and the Style Panel on the right side:
If you'd like to position or resize the panels in a way that works better for your personal workflow, just click the lightning bolt icon in the top-left corner of the Campaign Builder.
With Auto Layout turned off, you can re-position and resize the panels in any which way you'd like.
To automatically return the panels back to their default positions, just click the lightning bolt icon once again to turn Auto Layout back on.