How can I make background of block be transparent?

I can do it using a work around like this

.left-block-content {
background: url("img/menu.png") ;
background: #;
border: 1px solid #;
padding: 10px;
color: #333;
height: 224px;

removing code numbers for color
but how would be the proper way to do this?

Also is there a simple way to change a theme so it is 100% of screen I found some themes that are like this but not many and none have features I want.
I have been looking through css files trying to figure out what the differences are to be able change a theme. But so far no luck I just know enough about code, learning as I go.
Any help for getting me into right area?
Thanks

Hi Mike,
If you are doing it in a css file, try using

background:transparent;

this will affect all blocks in the column container in which those blocks are used