One of the interesting things built into the rendering system of Yii 2 is the support for themes. Together with the amazingly versatile asset bundle system, Yii 2 gives another level of control over the appearance of the web application under maintenance.
The definition of a theme from the official documentation and DocBlock in the comments is probably the best explanation of this concept ever. Here it is:
A theme is a directory of view and layout files. Each file of the theme overrides corresponding file of an application when rendered. A single application may use multiple themes and each may provide totally different experience. At any time only one theme can be active.
The crucial part is that a theme is a separate set of view files that ...