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.
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:
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.
The "UI Elements" page gives specific style controls over different elements of the UI.
For further customization, Custom CSS is available on the Agency plan.