Patterns & Practices (PnP) team released a ton of open source examples for using CSOM to do all sorts of fun things in SharePoint including uploading and applying branding to multiple sites.
There are certainly some pros and cons between using declarative sandbox solutions vs this CSOM method, but I’m still formulating my thoughts on what I would say for guidance.
Microsoft has stepped up and created this code that is blessed by the product team to upload and apply branding correctly with CSOM. Check demo for further details
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