You are able to change the design of your helpdesk easily by going to Admin> Help Center > Design. Here you can choose a background theme. You are able to remove the image on the theme and change the colors to match your branding.
###Adding a logo to the Help Center:
You can add a logo to the helpdesk in the 'Portal Settings' section of the Support Portal. This can be added in HTML to either the Portal Main Content section or the New Column Content Section.
Here is the example code for adding a logo: