To pick a theme for your form, click the Themes button at the top of the form and select your favorite theme.
Then, click Apply and edit the form.
If you'd like to use a form template with a theme, go to your Docs list, select New > From template..., and click Forms under 'Narrow by type,' on the left side of the page. Once you decide what template to use, click Use this template next to it, and click the Theme button. There are over 70 themes of all different colors and styles you can choose from. Select a theme and click Apply in the top left of the page.
Note: these themes won't appear within people's email clients (like Gmail), but respondents will see the themes if you send them a direct URL to the form.