Yes, you are provided with the tools to adjust any of the existing templates. Specifically, you can edit a template

• by entering your own background image,
• adjusting its opacity,
• and selecting the way you would like it to be positioned or resized, in order to cover the background of your poster,

as is explained in the corresponding tutorial: Inserting and adjusting your own background images.

Furthermore, you can adjust the fonts and colors according to your liking, as is detailed in Adjusting the typography.