When creating an HTML page, the equivalent of leading is the CSS "line-height" property, used to set the amount of space between each line. The line-height can be set to a fixed height, 12 pixels ...

Paste the following code into that box:p { line-height: 300%; }This CSS code tells Tumblr to set the line height of all your paragraphs to 300 percent. You can set this value to anything you like.