Merge workbench (works without the extension)
Pick a synced portal on the left (or paste theme rules), paste the portal's current Head Markup CSS, and merge: on a selector match the theme wins and everything else is kept; anything new is appended. Conflicts that can still beat the theme are flagged.
Theme library
Use the extension for live capture, then manage client and seasonal variants here. Duplicate a design, rename it, activate the one the team should pull, or load a theme into the merge workbench.
Theme CSS
History
Every sync and theme update keeps a snapshot (last 20 per theme). Click one to see what changed; restoring snapshots the current state first, so a restore is itself reversible.
Portal's existing head CSS
Merged result
The deploy kit wraps this stylesheet as a Salesforce CLI project: deploy the static resource with one sf command, or paste the included head markup. Uses the merged result when you have run a merge, otherwise the theme CSS as-is.