How do I change the colour and opacity of the dark gradients?

You can use CSS to override the default colour and opacity. Go to Theme Options > Styling and in the CSS Code box add:

.gp-top-bg-gradient-overlay {
background: -moz-linear-gradient(top, rgba(0,0,0,0.95) 0%, rgba(0,0,0,0) 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(0,0,0,0.95)), color-stop(100%,rgba(0,0,0,0)));
background: -webkit-linear-gradient(top, rgba(0,0,0,0.95) 0%,rgba(0,0,0,0) 100%);
background: -o-linear-gradient(top, rgba(0,0,0,0.95) 0%,rgba(0,0,0,0) 100%);
background: -ms-linear-gradient(top, rgba(0,0,0,0.95) 0%,rgba(0,0,0,0) 100%);
background: linear-gradient(to bottom, rgba(0,0,0,0.95) 0%,rgba(0,0,0,0) 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#f2000000', endColorstr='#00000000',GradientType=0 );
}

.gp-bottom-bg-gradient-overlay {
background: -moz-linear-gradient(top, rgba(0,0,0,0) 0%, rgba(0,0,0,0.3) 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(0,0,0,0)), color-stop(100%,rgba(0,0,0,0.3)));
background: -webkit-linear-gradient(top, rgba(0,0,0,0) 0%,rgba(0,0,0,0.3) 100%);
background: -o-linear-gradient(top, rgba(0,0,0,0) 0%,rgba(0,0,0,0.3) 100%);
background: -ms-linear-gradient(top, rgba(0,0,0,0) 0%,rgba(0,0,0,0.3) 100%);
background: linear-gradient(to bottom, rgba(0,0,0,0) 0%,rgba(0,0,0,0.3) 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#00000000', endColorstr='#99000000',GradientType=0 );
}

.gp-side-bg-gradient-overlay {
background: -moz-linear-gradient(left,  rgba(0,0,0,1) 0%, rgba(0,0,0,0) 20%, rgba(0,0,0,0) 80%, rgba(0,0,0,1) 100%);
background: -webkit-gradient(linear, left top, right top, color-stop(0%,rgba(0,0,0,1)), color-stop(20%,rgba(0,0,0,0)), color-stop(80%,rgba(0,0,0,0)), color-stop(100%,rgba(0,0,0,1)));
background: -webkit-linear-gradient(left,  rgba(0,0,0,1) 0%,rgba(0,0,0,0) 20%,rgba(0,0,0,0) 80%,rgba(0,0,0,1) 100%);
background: -o-linear-gradient(left,  rgba(0,0,0,1) 0%,rgba(0,0,0,0) 20%,rgba(0,0,0,0) 80%,rgba(0,0,0,1) 100%);
background: -ms-linear-gradient(left,  rgba(0,0,0,1) 0%,rgba(0,0,0,0) 20%,rgba(0,0,0,0) 80%,rgba(0,0,0,1) 100%);
background: linear-gradient(to right,  rgba(0,0,0,1) 0%,rgba(0,0,0,0) 20%,rgba(0,0,0,0) 80%,rgba(0,0,0,1) 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#000000', endColorstr='#000000',GradientType=1 );
}

.gp-bg-overlay-dark {
background: rgba(0,0,0,0.8);
}

.gp-bg-overlay-light {
background: rgba(0,0,0,0.5);
}