Hey there louiskzx,
This can be done with some custom CSS. Try adding the following CSS code in the style.css file of your child theme or add it in your site using the following plugin.
http://wordpress.org/plugins/simple-custom-css
#wrapper {
min-height: 850px;
}This will set the minimum height for the page no matter the amount of content.
As for the color transparency, actually the background is using transparent image rather then color. You can achieve the same effect with using black color. Add this CSS code:
.page-id-15 .header, .page-id-15 #content, .page-id-17 .header, .page-id-17 #content {
background: rgba(0, 0, 0, 0.8) !important;
}The last numeric value in rgba (0.8) represents transparency, you can set the value between 0-1 where 0 is completely transparent.
This will change the transparency for only about us and contact us pages.
Have a great weekend :)
Best regards,
Bojan