If you are trying to customise a product page template on Horizon — for example moving the buy button onto the same row as the quantity selector, or adding a colour picker to a block — you will likely hit a frustrating problem: your changes show up in the HTML but the CSS never updates.
We ran into this ourselves. The block code was correct and the HTML was being served fresh, but the compiled stylesheet (compiled_assets/styles.css) was stuck on the old version. Hiding and showing the section updated instantly, which proved it was not a browser cache problem. The real cause is that edits made via the MCP API update the Liquid files correctly but do not trigger Shopify's CSS rebuild step — so the compiled stylesheet never gets a new version number and Cloudflare keeps serving the old one.
We tried several workarounds on Horizon but could not reliably force a CSS rebuild without going into the theme editor manually after every change, which defeats the purpose.
Our solution: we switched the store to Dawn. Dawn does not use a pre-compiled CSS bundle in the same way — styles are closer to the templates and update immediately when files are patched via the API. Everything that failed on Horizon worked on Dawn on the first try.
If you are on Horizon and hitting this, the practical options are:
1. Make CSS changes manually in the theme editor after patching via Claude — Claude handles the Liquid, you save the CSS once in the UI to trigger a rebuild.
2. Switch to Dawn or another theme that does not rely on a compiled asset bundle.
Happy to answer questions if you are stuck on something specific.