Please note that all credits go to the original contributors. While this includes me partly, my contribution is rather minor. Most of the work was done by http://meta.stackoverflow.com/phils!

What it does:

• light color of the topbar
• fix the topbar location (always floating on the top of the page, even when you scroll down)
• adjust the size of the dropdown-boxes (inbox, achievements) plus search-box and their font-size

What is contained, but commented out:
I've included some additional CSS which you can simply enable by removing the surrounding comment enclosures. To not have that reverted with each style update, you alternatively can copy it to your own UserStyle:

• Completely hide the avatar
• Alternatively: have it replaced by your username
• Hide achievements icon when there are no news
• Use a colorized SE logo instead of the gray one
• Shrink the search box, and let it expand automatically when it's focused

I'd prefer to have it in a way you could elegantly toggle it via some menu, but that's not possible with CSS unfortunately, and thus would require a UserScript.

What sites it supports:

• AskUbuntu
• StackOverflow (incl. Meta)
• *.stackexchange.com (not completely tested; works fine on Android.SE and dba.SE at least)
• Super User
• Server Fault
• StackApps

I've added some comments to the CSS to make it easier for you to adjust to your personal preferences, e.g. hide some elements or use a colored SE logo.

I cannot promise to keep it updated...