bug#45068: [PATCH] Modus themes 1.2.0

[Gregory Heytings <gregory@heytings.org>]

>>> +(deftheme modus-vivendi
>>> +  "Accessible and customizable light theme (WCAG AAA standard).
>>                                   ^^^^^
>>                                   dark
> 
> [ Answer only if it is easy: how do you draw those ^^^ below the text? ]
>

Here's a more polished version, which takes care of the prefix / 
fill-prefix, and works on multiple lines:

(defun undercaret (&optional arg)
   (interactive "p")
   (let* ((begin (if (region-active-p) (region-beginning) (line-beginning-position)))
          (end (if (region-active-p) (region-end) (line-end-position)))
          (lines (- (line-number-at-pos end) (line-number-at-pos begin) -1))
          (comment (and (/= arg 1) (= lines 1)))
          (final-forward-line -1))
     (goto-char begin)
     (dotimes (i lines)
       (let* ((line-begin (if (zerop i) begin (line-beginning-position)))
              (line-end (if (= (1+ i) lines) end (line-end-position)))
              (begin-column (progn (goto-char line-begin) (current-column)))
              (prefix-begin (line-beginning-position))
              (prefix-end (progn (beginning-of-line-text) (point)))
              (prefix-end-column (progn (goto-char prefix-end) (current-column)))
              (delta (if (< begin-column prefix-end-column) (- prefix-end-column begin-column) 0))
              (prefix-string (buffer-substring-no-properties prefix-begin prefix-end))
              (prefix (if (string-blank-p prefix-string) "" prefix-string))
              (whitespace (make-string (- (+ begin-column delta) (length prefix)) ?\ ))
              (do-under (< delta (- line-end line-begin)))
              (under (if do-under (make-string (- line-end line-begin delta) ?^) ""))
              (under-string (concat prefix whitespace under "\n")))
         (forward-line 1)
         (if do-under (insert under-string) (setq final-forward-line -2))
         (setq end (+ end (length under-string)))
         (when comment (insert prefix whitespace "\n"))))
     (forward-line final-forward-line)
     (goto-char (line-end-position))))