I don’t like the new twitter design update, but the goods news are that it is very easy to fix. My biggest complaint is about the width of navigation bar – it is wider than the main content and it doesn’t make any sense. IMO the width should be the same for both navigation bar and content in this case.
Stylish, a browser extension for Chrome & Firefox will help us to change the design with just few lines of CSS. You need to add a new rule in Stylish, which applies to a domain twitter.com, and add style rules which you can see below. Check code comments for clues what the code does.
This is how it looked before: