QGIS 2.12 now supports user interface theming support which you can use to customise the appearance of window backgrounds, buttons etc. By default we ship with two themes: Default and Night mapping. The latter is a dark theme which some people may prefer if they find that light themes cause eye strain. If you know a little CSS you can create your own custom themes fairly easily too...
Find out more about theming support by reading Nathan Woodrow's blog article.
This feature was developed by: Nathan Woodrow