Add a "Copy Colors" button to Popper

[TerabyteTiger]

Add a button to the bottom of the Popper popout to copy theme to clipboard.

I.e. Clicking this button while the default-light theme is selected would copy

.default-light {
  --primary: #351c4d;
  --secondary: #fff;
  --accent: #f5ab99;
  --link: #674672;
  --text: #351c4d;
}

[mblancodev]

I would like to take this

[TerabyteTiger]

Functionality added by #28