Copy button in code blocks - low contrast in light mode

[ysthakur]

In light mode, the copy button in code blocks has very low contrast. I assume the color used for the button is the same as the text color, which is white in dark mode but black in light mode:

When I hover over it, it has slightly better contrast due to the gray background, but it's still hard to see:

This also happens with ansi blocks, which use One Dark Pro rather than Dark Plus.

It should be possible to use white for the button foreground no matter what. Another, less preferable, option would be to use a light theme for code blocks in light mode (e.g. light-plus by default and one-light for ansi), but that would require vetting a new theme.

I can take this issue.