theme variables

OH,

Here it is in the ReadMe file.

6. In your custom block templates, you can also use values from the
$blocktheme_vars variable. This array contains custom variables defined in the
block edit form, right below the block theme selection.

my_var|value corresponds to $blocktheme_vars['my_var']

Also

use the following code to detect if the variable is defined to prevent errors.

if( isset($blocktheme_vars) ){

}