Microsoft released a tool for customizing Outlook Web Access.
Besides adjusting the theme, you can also adjust blocked file types ,smime settings and lots more..
The Outlook Web Access (OWA) administration tool provides web-based UI for all administrator tunable OWA settings. It provides a list of all servers in the domain and allows administration of OWA settings on all Front-end and Back-end servers. The tool ensures that settings are correctly written to the server’s registry and provides inline documentation for all configurable features.
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