You can change the colors of your form in Theme Settings.
Go to Theme – which is found up the top right in the form editor.
There you will see commonly used defaults. In Theme Settings, you can set your Background Color, Active Color, Text, and Warning colors and choose Fonts for your form. You can also export themes from existing forms.
In Typography, you can control the font, sizing, line heights and coloring of text types like H1, question etc. You can also create custom text styles, which can then be applied to any text on your form.
UI Elements section gives specific style controls over different elements of the UI, from the background to the submit and pagination buttons. Custom CSS is also available.