Well your style css can be carried over to photopost but you can not carry over html designs without altering Photopost templates
The bulk of what you want to do can be acheived by looking at the html of your phpbb forum and making the header and footer of photopost the appropriate things
header maybe something like this
Code:
Content visible to verified customers only.
footer maybe this
Code:
Content visible to verified customers only.
You may need to make sure to edit that avalance.css stylesheet and make any background images FULL URL to them instead of relative paths