Some images are difficult to see in dark mode

The content of some images is not shown correctly when using  GitHub's dark mode.
The following example is from [abi-aa/aapcs64/aapcs64.rst](../blob/main/aapcs64/aapcs64.rst):

<img width="609" alt="light_vs_dark_modes" src="https://user-images.githubusercontent.com/12090265/148647797-cf5d16cb-93e9-439d-b87b-6d60055860aa.png">
