[6.0] Riverlea - merge text-light and light-text

[ufundo]

6.0 version of #31994

cc @vingle

[civibot]

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[ufundo]

Similarly reinstating the crm-text-dark changes here.

[ufundo]

Noticed some issues with heading/warning text colours getting flipped (particular in dark mode). I think they are resolved now.

[ufundo]

Closing this as have redirected the original PR to 6.0 now: #31994