Yes, you can create or edit your custom themes in VWO Surveys.
1. Login to your VWO account.
2. Go to INSIGHTS > Surveys.
3. From the Surveys dashboard, select the one you want to work with.
4. Go to SETTINGS > OPTIONS and from the Theme dropdown, select New Theme.
5. The CSS for the selected theme will load in the code boxes. You can edit and update the existing theme, or save it as a new theme.