fix(live-samples): make Copied! message visible + use theme

[OnkarRuikar]

Summary

Fixes #5537

Code section "Copied!" message is not visible for multiline code sections.
Change the message foreground and background colors according to theme.

Problem

The message was staying behind the code section due to z-index set to -1000.
Colors are hardcoded so they don't change as per theme.

Solution

Remove z-index property from the CSS rule ..copy-icon-message.
Also set color using CSS vars.

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Screenshots

Before

The message was not visible.

After

1. Light Theme:
   

2. Dark Theme:
   

[OnkarRuikar]

@caugner First commit was not signed. So I signed it later.
Do I need to delete the first unsigned commit?
Or do I need to make a brand new pull request?

[caugner]

@OnkarRuikar Here's what I would do:

# Switch to branch (and make sure you don't have any uncommitted changes).
git checkout fix-#5537
# Reset the branch.
git reset --hard main
# Cherry-pick your signed commit.
git cherry-pick 82d455e3764b954729a950403741f76f61e0418a
# Overwrite the remote branch.
git push --force

edit: Afterwards this PR should only have one signed commit. No need to create a new PR. :)

[caugner]

LGTM, I will merge once you have followed the steps to reduce the branch to a single signed commit (if you can).

[OnkarRuikar]

@caugner done Thx!