fix case for icon theme

The theme name should be all lowercase IMHO:

    root@rotten-fruit share/themes# grep CursorTheme */*theme
    Aldabra/index.theme:CursorTheme=mate
    BlackMATE/index.theme:CursorTheme=mate
    GreenLaguna/index.theme:CursorTheme=MATE
    Menta/index.theme:CursorTheme=mate
    TraditionalGreen/index.theme:CursorTheme=MATE
    TraditionalOk/index.theme:CursorTheme=MATE