One of the large benefits of using a GTK+ (or Qt)-based API is that the widget set can be themed. Users are able to install any number of themes (or write their own) to control how applications look. While this can add a testing overhead, they will behave the same across all platforms so the burden is somewhat reduced.

Let's see a few different themes applied to our GoMail application illustrated here, starting with a great light theme named Clearlooks.

• The Clearlooks theme on Linux:

• Compose in Clearlooks:

On Windows, the default theme looks more like the standard widgets, though the user can load any other GTK+ theme. Notice that the default icons are also different, more in-keeping with the operating system standards.

• The Windows default theme:

• Compose with Windows:

There are also many dark themes; Arc Dark is very popular.

• Arc Dark theme running on Linux:

• Ark Dark compose window:

Many themes are designed for nostalgia, including this CDE theme, which is based on a colorful desktop environment from the 1990s.

• Running a CDE theme for the old-school look:

• Composing in a CDE theme:

As you can see, the colors of the user interface elements can vary significantly but the layouts are largely consistent. If you look at the buttons (Send and Cancel on the compose window), there's also a difference between how rounded some edges are. Applications built with Go-GTK should work well with any theme loaded, but it is advisable to check various different configurations as part of your quality assurance process.