Yes. To customize the appearance of your survey, select New Theme in the themes dropdown. This will load the CSS of the default Dark theme. If you want to customize a different theme, just click on the theme dropdown and select the theme and that that themes CSS will be loaded ready for editing.
The CSS is displayed in two code boxes - one for the Survey container (the actual survey box which appears and contains form elements such as buttons, text boxes, etc.) and the other for the contents of the survey viz. the question, etc. You can customize the survey as you wish and verify by checking the preview on the right. Once you are convinced, you can save the new custom theme you created for reuse later.
For the duration of the public beta, this feature is available for all accounts. After public beta ends, this feature may or may not be available in all plans.