If you have been able to develop your own css styles then you can update the css stylesheet within the theme folder with your custom styles.
You will have to use other resource from the modeler like containers and layout grids in in conjunction with your css styles in order to get your desired look.
Unfortunately dropping a theme in from Photoshop directly on a web app is not possible in Mendix.
See this tutorial on how to customise your theme.
Hope this helps