Forms doesn't have an easy way to customize the look and feel so I've decided to use the IBM Http Server to expose a different custom CSS file of the login page instead the standard one.
IBM Http server >= 8.5.5 FP1If you have at least IHS 8.5.5 FP1 you can use the following directive
SetEnvIf Request_URI ^/forms/open/126.96.36.1992/freedom/css/application_login.css skipwas=1
Reding this directive IHS will be instruct to search the application_ligin.css file in same place but ignoring the IHS Plugin.. so you have to replicate same folder on local document root with the css file.
IBM HTTP < 8.5.5 FP1
Using an old version of IHS I've used the rewrite module
Alias /static/ "/opt/IBM/HTTPServer/www/static/"
RewriteRule ^/forms/open/188.8.131.522/freedom/css/application_login.css$ /static/application_login.css [PT]
The alias directive is an optional facility in case we will need same folder structure for others file in the future.
This method with rewrite rule is also available on IHS >8.5.5 FP1