The default white background of your question box can only be changed using custom CSS on our Agency plan. You can remove this background altogether though, no matter which plan you're on.
To remove question backgrounds, go to Theme → UI Elements → Questions and toggle off "Has background and shadow."
The boxes of selected answers (e.g. for Multiple Choice or Product questions) are governed by the Active Color. Both the color and transparency can be customized to whatever you'd like under Active Color in the Theme Settings section.
The font color for typed answers matches the Active Color by default. If you would like to change this text to be a different color, you can change it under Theme → Typography → Answers.