I’ve been using material-theme lately, and I sometimes switch around, but I’ve found that solarized produces the best exported code block results. To avoid having to remember to switch themes when exporting, I wrote a quick wrapper for org-export to do it for me:

(defun my/with-theme (theme fn &rest args)
  (let ((current-themes custom-enabled-themes))
    (mapcar #'disable-theme custom-enabled-themes)
    (load-theme theme t)
    (let ((result (apply fn args)))
      (mapcar #'disable-theme custom-enabled-themes)
      (mapcar (lambda (theme) (load-theme theme t)) current-themes)

(advice-add #'org-export-to-file :around (apply-partially #'my/with-theme 'solarized-dark))
(advice-add #'org-export-to-buffer :around (apply-partially #'my/with-theme 'solarized-dark))

Voilà, no more bizarrely formatted code block exports from whatever theme I might have loaded at the time :)