From BlenderWiki

The Window Header

Most windows have a header (the strip with a lighter gray background containing icon buttons). We will also refer to the header as the window toolbar. If present, the header may be at the top (as with the Buttons window) or the bottom (as with the 3D View window) of its window’s area.

Sample window headers using the default theme.
Sample window headers using the “Rounded” theme.

If you move the mouse over a window, its header changes to a lighter shade of gray. This means that it is “focused”: all hotkeys you press will now affect the contents of this window.

Hiding a header

There are two ways to hide the header from a window.

You can hide a particular window’s header by moving your mouse over it, and clicking RMB to display the Header popup menu – select its No Header entry to hide the window’s header.

Another method of hiding a particular window’s header is to move your mouse over a dividing frame/border of this window. When the mouse cursor is positioned correctly, it will display as double-arrows.

When the double-arrows cursor is displayed, RMB click. A popup menu will be displayed – select its No Header entry to hide the window header. Note that with this method, it is the header of the active (focused) window which will be hidden (i.e. the window from which comes your cursor…).

Showing a header

Once a header has been hidden, to redisplay it, do the following:

Move your mouse over a dividing frame/border of the window in which you want to show a header. When the mouse cursor is positioned correctly it will display as double-arrows.

When the double-arrows cursor is displayed, click RMB or MMB . A popup menu will be displayed – select its Add Header entry to add the header back to the window. Here again, you must be sure that the active/focused window is the one you want to show the header in, when clicking RMB on the border…

Header position

The header can be displayed at the top or bottom of its frame. To set a header’s position, RMB click on it and choose Top or Bottom from the Header popup menu.

Theme colors

Blender allows for most of its interface color settings to be changed to suit the needs of the user. If you find that the colors you see on screen do not match those mentioned in the Manual, then it could be that your default theme has been altered. Creating a new theme or selecting/altering a pre-existing one can be achieved by clicking on the Themes button of the User Preferences window. The User Preferences window, Themes section selected.

Window type button

The icon at the left end of a header, with a click of the LMB , allows selection of one of the sixteen different window types.

Menus and buttons

Most headers exhibit a set of menus, immediately next to this first “window type” button. Menus allow you to directly access many features and commands. They can be hidden and shown via the small triangular button to their left.

All menu entries show the relevant hotkey shortcut, if any.

Menus and buttons change with the window type, the selected object type, and the active mode. They show only actions which can be performed.