@rg94LM - You can’t change it with CSS.  It is an attribute (value), not a format.

Alternatives:

• Look & feel → General
• Edit block → Format (to change for a specific block)
• JavaScript: jQuery("#NextButton").val("Next");

Thank you TomG! You have helped me with so many other issues on this forum, pretty neat to see you answer my first Q here!

Do you know if this is possible to do for the whole survey (not just the block)? Or even for a custom theme? 

Thank you for your help!

Look & feel → General will do it for the entire survey. 

Next/Previous buttons aren’t options that can be set in custom themes.