Survey "Back", "Next", "Done" and "Cancel" button can be customised by updating the survey module button image files.
Note that this will affect the buttons across all surveys in the system regardless of the survey template they are using.
The survey module custom button image files are located inside exchange-sites at "\_Theme\Survey\Resources\Images"
There are two buttons "bg_button_a.gif" and "bg_button_span.gif" which contain the up and down states of the button and which are designed to accommodate the different amounts of text for different button translations.
These buttons should be replaced with your new button style. You will need assistance from a Vuture administrator to make this change.
There is also a custom stylesheet at "\_Theme\Survey\Resources\StyleSheet\Custom.css" that can be edited if extra styles are required in the survey.
The original button images are attached to this page in case you need to reset them.