To change the Theme of your survey, go to:
Login » Communities » (Select Community) » Edit »Settings>>Display Settings(right hand side)
Under Themes tab, select the theme using the arrow keys.
You may click on Preview to check the theme and click on Save Canges
To change the logo, go to:
Login » Communities » (Select Community) » Edit »Settings>>Display Settings(right hand side)>>Banner
Under Banner, select the logo from Website Logo option.
You may upload an image for your logo, or also select the image from My Image Library