div.A {border: thick black solid; background-color:red !important}
and then
{border: 1.5ex #66f solid; background-color:#6f6} /* There must not be a space between 1.5 and ex */

p.A(1) inside div.
{border: medium #f06 dotted; background-color:yellow}

p.A(2) inside div.
{border: medium #f06 dotted; background-color:yellow}

p.A#bigmargin (3)

p.A(4) inside div.
{border: medium #f06 dotted; background-color:yellow}