over riding css dynamically using jquery

over riding css dynamically using jquery

is it possible to override css dynamically??
Like in css we specify
.mainDiv{
height:100px !important
}
is it posssible to do the same with jquery css
Ive tried $(".mainDiv").css({"min-height" : ($(window).height() * 0.7)
+"px !important"}); This wont work..