Firstly, I would encourage you to update to v5.0 (& it comes with more themes, too).
Concerning the themes in general: By default, the theme styles apply to all defined forms, unless specifically defined via custom CSS. Since every form also has its own ID the generic CSS can be easily turned into form specific CSS.
This way every form could have its own look & feel, but again all code has to be in one CSS theme file. So you'd have to do a bit of copy and paste and customization to achieve that.
The theme with the "get in touch" header, uses an image. If you'd like to replace the image with your own, you can simply replace it with whatever you'd like to be displayed. Or you could remove it completely by adjusting the CSS (remove the respective background: attribute for the fieldset element)