The requirements for this tutorial: Firefox or Chrome for a browser, go to this site and install Stylish and the tumblr - Clean Slate theme.

The Big Background is the area of tumblr behind all other content and controls on your dashboard. Modifications to the Big Background will depend upon what you want to see and how it will function.

One Image, One Screen
This layout is for one image, filling the background, which will remain in place while scrolling.

The instructions that follow display an image of what changes will occur after editing, the line of code which requires the edit, and any additional notes.
 

The CSS Code - Editing Instructions

The generic blue tumblr, with no style applied

Selecting a Background Image

• Find the URL for the image you have selected for the background.
• Paste the URL between the quotes and brackets of the following line:
  

  background:url("<PASTE YOUR URL HERE>") no-repeat fixed !important;

The Edited Code

/*BIG BACKGROUND begin - background for the entire screen*/
 body {
   background:url("<PASTE YOUR URL HERE>") no-repeat fixed !important;
   background-color: rgba(44,71,98, 0.95) !important;
   background-position: 0% 0% !important;
   background-size: 100% 100%  !important;
 }
 /*BIG BACKGROUND end*/

After background URL selected and edited

Setting Content Pane Opacity

• Set the desired opacity level (transparency) for the content pane by changing the last number.
  
  • minimum: 0.00 = transparent / completely clear
  • maximum: 1.00 = opaque / completely solid
    
    The example has an opacity level set to 0.25 and looks like this after editing:
    

    background: rgba(44,71,98, 0.25) !important;

The Edited Code

/*CONTENT begin - background for posts, icon bar, sidebar*/
 #content {
   background: rgba(44,71,98, 0.25) !important;
 }
 /*CONTENT end*/

After editing Content Opacity

Setting Icon Bar Opacity

• Set the desired opacity level (transparency) for the icon bar background.
  
  • If a content pane rests on top of another pane with opacity, the two levels and colors are combined.
    
    The example has an opacity level set to 0.25 and looks like this after editing:
    

    background-color: rgba(255, 255, 255, 0.25) !important; 

The Edited Code

*ICON BAR BACKGROUND begin - background for the icon bar*/
 #new_post {
   background-color: rgba(255, 255, 255, 0.25) !important;
 }
 /*ICON BAR BACKGROUND end*/

After editing Icon Bar Opacity

Setting Post Content Opacity

• Set the desired opacity level (transparency) for the post content background.
  
  • If a content pane rests on top of another pane with opacity, the two levels and colors are combined.
    
    The example has an opacity level set to 0.25 and looks like this after editing:
    

    background-color: rgba(255, 255, 255, 0.25) !important;

The Edited Code

/*POST BACKGROUND begin*/
 #posts LI[class*=" post"] {
   background: rgba(255, 255, 255, 0.25) !important;
 }
 /*POST BACKGROUND end*/

The image for this stage displays only the changes after editing the above lines.

After saving and enabling the completed style, this is the final result. The remainder of the unedited code should clear up the blue/green buttons and the frame around the radar.

If you are feeling really adventurous, there are also places to change the following within this style:

BIG BACKGROUND
ICON BAR
ICON BAR BACKGROUND
AVATAR ARROW
CONTENT
POST BACKGROUND
SELECTION ARROW
GREEN BUTTONS
RADAR
NEW CONTENT

Please feel free to ask if you have questions or need help.