What I'm currently using for the ACP is a Bootstrap base. There are two SCSS files that are compiled using Gulp, combined with the bootstrap scss file, and minified into one stylesheet CSS. You can find a current working copy of that in my repository:
https://github.com/marc1706/phpbb/tree/ ... p_redesign
Most of the ACP style files are actually here: https://github.com/marc1706/phpbb/tree/ ... yle/assets
GIF giving an overview of the general views and the top menu:
http://imgur.com/5pgtVQo
Current repsonsive view with sidebar:
I do also agree that we should try to move forward from prosilver, something we probably should've already targeted for 3.1. There are a lot of new tools and design approaches out there nowadays. We should take these opportunities and improve the usabilitiy and overall look of phpBB on the frontend. hanakin has already put some thought into this and I think with the community we'll be able to create something that will put phpBB into the current times.