changing background colors of grid rows dynamically in extjs 4.2.2

Figured out what the Problem was.

Because I am using a theme I had to put the custom CSS File before the standard ExtJS CSS with the "!important" flag.

New css file:

.later .x-grid-cell {
        background-color: #FFB0C4 !important;
    }
.now .x-grid-cell {
        background-color: #5491BD !important;
    }