Add color to time slots

[raywo]

As a user I want to visually distinguish time slots by their color. Use a grey color palette for this which represents morning, noon, afternoon and so on.

$morning-dawn: #E8E8E8;    // Morgengrauen
$morning: #D6D6D6;         // Morgen
$noon: #BFBFBF;            // Mittag
$afternoon: #A9A9A9;       // Nachmittag
$evening: #7F7F7F;         // Abenddämmerung
$night: #595959;           // Nachtbeginn