Codex/fix gatekeeper app accent theme

[deloreyj]

After setting up my Cloudflare OS instance with a custom accent color, I noticed that the primary call to action buttons in the Schedule and Context tabs were using the default orange. This pull request propagates the accent colors into the built-in gatekeeper iframe.

The change is narrowly focused on the accent color and follows the same pattern as the theme subscription.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[deloreyj]

I have read the CLA Document and I hereby sign the CLA

[Maximo-Guk]

Would it be possible to provide some before and after screenshots of your change?

[ndisidore]

This is looking good to me! I looked briefly into this and came to a similar changeset.
Agree before/after pictures would be nice to have

[deloreyj]

Sure thing! cc @Maximo-Guk and @ndisidore

Before: New collection and create buttons button are default Cloudflare Orange despite the app theme being changed. However the New Workspace button correctly adopts the new theme

After: The app passes through the user-specified app theme to the built-in gatekeepers' iframes

[ndisidore]

@ask-bonk review this

[ndisidore]

Checking this out and testing locally I found a few issues e.g. ServerConfigContext, which is likely only populated at boot, while the admin color picker updates the host DOM directly

Also I think it probably makes sense to roll into the single theme RPC:
I took a stab at what I think may be a better approach here: #155 utilizing useSyncExternalStore