Bedrock provides two default variables: htmlClass and bodyClass. When you want to set an HTML class for every page, simply go to _layouts/master.pug and find these lines:

block pageVariables
    //- Use this block to append page variables in templates
    - var projectTitle = "Bedrock"
    - var htmlClass = ""
    - var bodyClass = ""

Now you can change the empty string to something that you like.

If you want to only add a certain body class in a specific template, use Pug’s template inheritance to append to the block. For example, let’s say you already have a utility class to set full height on the pageVariables block in master.pug, but you want to set a specific body class on another page, it will look like this.

In master.pug:

block append pageVariables
    - var htmlClass = "u-full-height"
    - var bodyClass = "u-full-height"

in your template (do repeat the original class value):

block append pageVariables
    - var bodyClass = "u-full-height your-specific-class"

You can use this technique to set global variables or page-specific variables; or even page-specific variables that inherit to deeper templates. You could probably be more clever than this code and append to the original value too. Anyone knows how?