As noted in the section about modus-themes-org-blocks, Org contains a
variable that determines whether the block’s begin and end lines are
extended to the edge of the window (Option for org-mode block styles).
The variable is org-fontify-whole-block-delimiter-line.
Users who change the style of Org blocks from time to time may prefer to automatically update delimiter line fontification, such as with the following setup:
(defun my-modus-themes-org-fontify-block-delimiter-lines ()
"Match `org-fontify-whole-block-delimiter-line' to theme style.
Run this function at the post theme load phase, such as with the
`modus-themes-after-load-theme-hook'."
(if (eq modus-themes-org-blocks 'gray-background)
(setq org-fontify-whole-block-delimiter-line t)
(setq org-fontify-whole-block-delimiter-line nil)))
(add-hook 'modus-themes-after-load-theme-hook
#'my-modus-themes-org-fontify-block-delimiter-lines)
Then M-x org-mode-restart for changes to take effect, though manual intervention can be circumvented by tweaking the function thus:
(defun my-modus-themes-org-fontify-block-delimiter-lines ()
"Match `org-fontify-whole-block-delimiter-line' to theme style.
Run this function at the post theme load phase, such as with the
`modus-themes-after-load-theme-hook'."
(if (eq modus-themes-org-blocks 'gray-background)
(setq org-fontify-whole-block-delimiter-line t)
(setq org-fontify-whole-block-delimiter-line nil))
(when (derived-mode-p 'org-mode)
(font-lock-flush)))