Fix colour variables

[trallard]

While working on the design system documentation, we noticed some fixes needed:

• Fix heading colour definition: Use same colour as body
• Add note about zindex: Closes Missing variable: zindex-tooltip #1953
• Replace the undefined variable with PST variable: we were using --pst-color-panel-background for the background of the card (introduced in Add panels support #641, but it seems this was removed later and it was not properly replaced). Since cards are meant to give some sense of elevation, I replaced this with --pst-color-on-background, which mostly applies to the dark theme (see screenshots below)

Current colour:

In this PR:

light theme - it's the same before and after as we rely on shadows for elevation

cc @smeragoel

[github-actions]

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