Problem default CSS

I have a problem with the default Markdown css, ie in Preference->Media the Stylesheet field is blank.
Using Dark mode in macOS Preference/General the #4 Header (#### Header) tag displays in dark violet color on black background, all the other header formats (1,2,3,5,6) are shown as white text on black background. Where can I change this without setting a dedicated .css file in Preference->Media?

The only possibility is actually a custom CSS file. Or to disable dark backgrounds of documents (see Preferences > General > Appearance). However, over here it looks as expected. How does the source of the document look like?

Simple testfile;

### 11:07:23

Testar hur headers ser ut utan CSS-fil (default)

# Header 1
## Header 2
### Header 3
#### Header 4
##### Header 5
###### Header 6

that’s it.
I’m really playing around with css because it seems to have an impact in the operations of DEVONthink To Go beta 3.0. Using different CSS-files on the Mac, results in different errors in DEVONthink To Go.

This was not helpful. How do I show content of file?

The headers are white as expected and the links of the table of contents are also correct:

Therefore the only possibility is indeed to use a custom CSS if you want to change the color of the links.

My view

Which version of macOS do you use? A screenshot of Preferences > Media might be useful too.

macOS 10.15.7

I can’t reproduce this, neither in dark nor light mode using the same macOS version & settings. Or do you use a custom style sheet in Preferences > Web?

Yes I did!
Removing this file and redesign is now OK.
Thanks for all the help!