In the download you will also find the source file. Open this in Photoshop there you can change the browse and send your message buttons. Color and text of those are both pictures.
The language parts you can find in en-GB.com_yjcontactus.ini in \language\en-GB.
The styling, the css, you can find in yjcontactus.css in \components\com_yjcontactus\helpers. But you can also edit the css from the backend in the contact us component, just what you prefer.