This is happening at https://tweakers.net/, a popular technology site in The Netherlands and Belgium.
In the header the account kind of icons become garbled. In the news overview, some items get a weird background behind the text. There are possibly other issues, but I suspect they share the same root cause. These issues appear to do with icons only, not pictures within article content.
Example before:
After:
Current workaround is to use the Filter+ theme.
I'm using Firefox 65 & the latest Dark Reader addon from the Mozilla repo.
Here is another example, for google docs equation toolbar:

With Filter or Filter+ modes this doesn't happen.
On Tweakers, this seems to be the case because Dark Reader applies the wrong image set.

Disabling the top one, so the responsive one, fixes the issue for me. The logo is even worse, as it picks a totally unrelated image:

Hi Medetweakers ;)
It has been fixed in the latest version of darkreader!
Regards,
Gusted
Most helpful comment
Hi Medetweakers ;)
It has been fixed in the latest version of darkreader!
Regards,
Gusted