fix: :bug: title color for code block in light mode

[flixlix]

Dark mode

in dark mode the title above the code block worked fine.

Light mode

In light mode, the title wouldn't appear, because the background was also white.

Selected Text

Here you can see that the text is still rendered in light mode.

[vercel[bot]]

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[ruru-m07]

@flixlix can you also add some images after resolving this issue soo we can see essely what's changes you make

[flixlix]

@flixlix can you also add some images after resolving this issue soo we can see essely what's changes you make

@ruru-m07 here you go:

[kachkaev]

@shadcn can you please review this one when you have a sec? This should be a quick for a pretty important UI / a11y issue in the docs:

https://ui.shadcn.com/docs/theming#list-of-variables