This "hack" (if you want to call it that) lets you change the default Roundcube Webmail v1.6 (Elastic skin) logo and favicon into whatever you want. It works so long as your replacement images are in the correct format. SVG and ICO.
In Apache, add the following rules, then reload
RewriteEngine On
RewriteRule ^/skins/elastic/images/logo.svg$ /path/to/new/logo.svg
RewriteRule ^/skins/elastic/images/favicon.ico$ /path/to/new/favicon.ico
.png)