A majority of the changes are not be visible; these tweaks go along with recent updates that were made to Stack Overflow:

• We have updated the site's CSS to a newly refactored LESS system, so that it's easier for us to fix SE network CSS bugs globally and launch new features in the future.
• We have updated some of the graphics to SVG for retina support.
• We have fixed a LOT of obscure bugs that arose when new features were not thoroughly tested across the entire network.
• We are launching the shiny new user profile!

Visually it will "feel" the same as the old site with slight layout adjustments.

This update should retro-actively fix most of the old design bugs. If you see any new bugs, or old ones not yet fixed, please post an answer here to let us know!

• I'm guessing it's too late but thought it might be worth a mention. Any chance something can be done about this at the same time? meta.gamedev.stackexchange.com/questions/1624/… – SpartanDonut Jul 16 '15 at 14:29
• Thinking about it chat probably isn't affected by the new changes but I don't know for sure :) – SpartanDonut Jul 16 '15 at 14:29
• @SpartanDonut chat isn't part of the update, but I will ping someone who might be able to make that adjustment. – Kurtis Beavers Jul 16 '15 at 16:46

I did some dev-tools digging in case it's useful: The button's font-family is hitting sans-serif, which on my system (and probably some other Linuxes), defaults to the generously-spaced DejaVu Sans.