bug#77841: Some svg image's black color is replaced by emacs's theme foreground color

[Eli Zaretskii]

> Date: Wed, 16 Apr 2025 20:09:46 +0800
> From: Billy Lei <LeiYiXia29@outlook.com>
> 
> I found this issue when debugging org LaTeX preview, see 
> https://list.orgmode.org/87cydchjqn.fsf@gmail.com/T/#ed7ba8310f82e8be8a2e7360df1568ed485434df3
> 
> > When using themes with background color set to #000000, and 
> > org-preview-default-process is set to dvisvgm, the LaTeX fragments are 
> > displayed incorrectly.
> > 
> > To reduplicate:
> > 
> > 1. Switch to a theme with background color set to #000000 (like Modus 
> > Vivendi)
> > 2. Set org-preview-default-latex-process to dvisvgm
> > 3. Create an org-mode document. Write some LaTeX fragment and press C-c C-x 
> > C-l to preview LaTeX fragment.
> > 4. The LaTeX preview is displayed as a white rectangle.
> > 
> > After debugging, I found that it is because dvisvgm omitts the default #000 
> > color value when generating SVG file, and emacs has a unique way of 
> > displaying the SVG, which caused foreground color to be applied to it.
> > 
> > A solution is to change org-format-latex-options to use transparent 
> > background by default. I've attached this fix.
> 
> It turned out that emacs displays svg images differently compared to an 
> image viewer. I can reduplicate this issue when I use emacs to view this 
> svg image, which emacs replaces the image's black color with the current 
> theme's foreground color:
> 
> https://commons.wikimedia.org/wiki/File:Flag_of_the_German_Democratic_Republic.svg
> 
> See also (Chinese): https://emacs-china.org/t/emacs-30-1-svg/29217

Yes, this is a feature.  I think to disable it, you need to include a
:background setting in the image spec.  Alan, am I right?