On September 2, 2014 Google announced custom themes. See [Your forms, your way](http://googledrive.blogspot.com/2014/09/custom-forms-themes.html) post on Google Drive blog.  

The steps from the related help article [Add a theme to a form - Docs Editors Help](https://support.google.com/docs/answer/145737?hl=en) are the following:  

> **Create a custom theme**   
> If you want even more control over how your form looks, you can create your own custom theme.
> 
>  1. Open the form.
>  2. Click Change theme in the toolbar.
>  3. Find one of the existing templates in the panel on the right to use
>     as a starting point and click Customize under the name of that
>     template.
>  4. Select a section of the form in the panel on the right to begin
>     editing that section. Some of the options you can customize include:
> 
>   
> 
>  - Adding custom images to the header and the page background
>  - Choosing font, font size, font color, and paragraph alignment for the    form text
>  - Choosing a color for the form and page backgrounds