Globex Designs, a Canadian company, has released a Firefox extension that completely changes the design of Google services using a lot of visual enhancements. It's called Google Redesigned and, at the moment, it supports Google Mail and Google Calendar. The extension uses local CSS files and remotely hosted images.
The possibility of changing how a website looks by inserting local CSS files has existed in Firefox for a long time, but it's not something common users adopted. This feature is mostly used by developers or people with various vision deficiencies, who need to see larger texts or change a particular color.
Because the styles were widely appreciated, the company decided to transform them in a full Firefox extension which offers better deployment and self-updates. Using this extension is very simple. You just need to download, install and enable it and you're pretty much set. There are other options like manually force update check, instead of the automatic daily check, disable a certain style, edit a style and bug submission. Even though it currently only supports the Google Mail and Google Calendar, the company plans to extend it to other services, too.
The use of this extension is not restricted to Firefox only. It works with any Gecko-based browser which includes Flock, IceWeasel etc.; however, use on a non Gecko platform is impossible because the styles rely on Mozilla-based CSS values.
The change of a website's layout through local CSS files is still a heavily debated issue as many developers feel users should not have such freedom, because it undermines their work and ideas, while others encourage this practice. On the other hand, a lot of users complained about the visually dull aspect, in their opinion, of certain popular websites or services, for which such extensions should make a real difference or even encourage them to create their own styles.
Here are some screenshots: