You can change the colors of your form from the Theme menu, which can be found by clicking the droplet icon in the top-right corner of the form editor.
Theme Settings
The first page of the Theme menu is called "Theme Settings." This gives you a basic overview of your form's style. From here, you can change:
Background Color — The background of all pages of your form, including the success page (shown after submission).
Active Color — Affects buttons, selected answers, and link text, as well as the default color for typed answer text.
Text Color — Sets the default color for all text on the form, except for answers, links, button text, and error messages.
Warning Color — Used for various error messages, like if submission is attempted when a required question isn't answered.
Colors 1-4 — These can optionally be used to save additional colors to your "color palette," so that you can easily apply them to specific elements on the following pages of the Theme menu.
Typography
You can fine-tune the styles of different text on your forms from the next page of the Theme menu, "Typography."
By default, the font color of each element will match the Text Color set in Theme Settings, with the exception of Answers, which default to the Active Color.
Users on the Pro or Agency plan can also create Custom Styles, which can then be applied to specific text on your form.
UI Elements
The "UI Elements" page gives specific style controls over different elements of the UI.
Pro and Agency users may also change the color of the submit and pagination buttons to something other than the Active Color (this is not supported for forms in Guided Mode).
For further customization, Custom CSS is available on the Agency plan.
Gotchas
The white boxes behind individual questions can't be changed to a different color, unless you target them using Custom CSS (Agency plan only). However, you can remove these boxes completely by going to Theme → UI Elements → Questions and toggling off "Has background and shadow."