It will work if you put those modifications to /etc/webmin/custom-lang file, as those strings are from global lang/ directory.
Additionally, you can style anything using Authentic Theme extensions editor (accessible from theme configuration page or navigation menu search).
For example to change Webmin icon color on login page, you could go with:
.session_login form .wbm-webmin {
color: #3668a9;
}
… to get a result as:
… as well you could use custom JavaScript code to alter HTML on the page anyway you need.