I also use rainbow-delimiters, but they are a bit too bright in my dark theme, so I:
(use-package rainbow-delimiters
:config
;; Darken them a bit
(cl-loop for f in (face-list)
if (and (string-prefix-p "rainbow-delimiters" (symbol-name f))
(face-foreground f)) do
(set-face-foreground f (color-darken-name (face-foreground f) 15))))
See also color-lighten-name
to go the other way.
org-modern is nice but its best feature (block brackets, IMO) is missing when org-indent is enabled; you could give org-modern-indent a try to get those back (disclaimer: o-m-i author).