Ulrich,
I may be completely off-base here but there are settings for controlling
what the LaTeX snippets look like and these may affect your case. I
used to use the following when I had emacs with a dark background:
#+begin_src emacs-lisp :results none
(setq org-format-latex-options '(:foreground "white" :background "black"
:scale 3 :html-foreground "Black" :html-background "Transparent" :html-scale
1.0 :matchers ("begin" "$1" "$" "$$" "\\(" "\\["))
org-latex-create-formula-image-program 'imagemagick
org-tree-slide-heading-emphasis t
)
#+end_src
Maybe play with these, specifically the first line?
--
: Eric S Fraga (0xFFFCF67D), Emacs 25.0.94.1, Org release_8.3.4-1049-g481709
