My setup

• Docker
• • I have tried the local windows installer - same core issue with Mixed Content
• 2 different proxy have been tried with the same Mixed Content issue.
• • Cloudflare
  • • Install the agent
    • configure the agent in the web configurator to http:localhost:8069
  • Nginx Proxy Manager
  • • Poke a hole in my firewall for ports 80/443
    • Use a script to update my dns locally (configured with my data)
    • create the cert for app.omnistreamerp.com and force SSL
    • create a proxy host app.omnistreamerp.com and point it to the static IP inside my container to the odoo webserver (docker compose setup)
• odoo.conf
• • add these entries and restart the odoo web container to apply settings
  • • proxy_mode = True
    • list_db = False
• In the database for table public.ir_config_parameter (or in the web UI settings enable dev mode > Technical > System Parameters)
• • web.base.url = https://app.omnistreamerp.com
  • web.base.url.freeze = True

When I then log into the web UI app.omnistreamerp.com and in APPS and when I activate "Website" all is good till I pick a theme/template - the page NEVER LOADS - the console shows mixed content. In another browser if I just hit app.omnistreamerp.com and do not log in (view it as a public user) the site loads perfectly FINE.

MIXED CONTENT only shows when picking a template/theme on initial setup AND/OR when one tries to edit the website then BAM you get MIXED CONTENT errors and chrome BLOCKS the site. This is NOT a proxy error this seams to be an Odoo application error with the community edition specifically. 

seams to me the proxy_mode = True setting is NOT doing its job.

if I hit view-source:https://app.omnistreamerp.com/ directly like this I can see the HTTP references - oddly this is ONLY a problem when you're either setting this up or you're trying to edit the site while being logged in - normal nonauthenticated browsing does not trigger this MIXED CONTENT problem - this IMO/IME does not seem like a proxy issue at all - given this behavior. Obviously you can add an exception to Chrome  / Edge to allow mixed content from app.omnistreamerp.com but that is not an acceptable workaround given this is an ERP system and it needs to be secure and work securely.

Setting the website domain in Configuration > Websites > selecting "My Website" has NO impact and the error also remains the same unchanged behavior. The only way to get this to work is allowing mixed content for this specific domain app.omnistreamerp.com in Chrome or Edge