style: plh_kids_teens_za theme updates

[jfmcquade]

PR Checklist

• PR title descriptive (can be used in release notes)
• Builds on [FEATURE] Create plh_kids_teens_za theme #3139. After that has been merged, this PR should be updated to target master

Description

Updates to the plh_kids_teens_za theme that were originally included as part of #3140. Having them in a separate PR here lets us test and iterate.

Git Issues

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[esmeetewinkel]

description	master	PR branch
button and header colour look strange together
plh_module_item colours have disappeared
radio button grid background colour has too little contrast with yellow emojis (not good on master either, feel free to propose something better)
nav buttons in module are very saturated (might be a matter of preference)

[jfmcquade]

@FaithDaka See this PR description – it includes the changes that were originally in #3140, which were addressing #3200.

Please could you either update this PR or create a new one to address #3200 directly. Some points to note:

• [CHORE] Refactor plh_kids_kw theme to use new theme colour vars #3099 has now been merged, which may have knock-ons for this PR (I believe the ZA theme is base don the KW theme directly)
• See @esmeetewinkel's comment above for a list of colour changes that probably aren't acceptable. We'll likely want to tweak this theme to override its *-variant colours, or else make a case for the changes from a design point of view