Ok, open up roundcube/skins/your_theme/templates/login.html (this is in /usr/share/apache2 in Debian) and pick something you want to change, say, the logo. You'll see:
<roundcube:object name="logo" src="/images/roundcube_logo.png" id="logo" />
Wrap it in template conditionals, accounting for each domain.
<roundcube:if condition="$_SERVER['HTTP_HOST'] == 'mail.domain1.com'" />
<roundcube:object name="logo" src="/images/domain1logo.png" id="logo" />
<roundcube:elseif condition="$_SERVER['HTTP_HOST'] == 'mail.domain2.com'" />
<roundcube:object name="logo" src="/images/domain2logo.png" id="logo" />
<roundcube:elseif condition="$_SERVER['HTTP_HOST'] == 'mail.domain3.com'" />
<roundcube:object name="logo" src="/images/domain3logo.png" id="logo" />
<roundcube:endif />
You might need to enable PHP in templates though the config file, I can't remember if PHP in conditionals counts as PHP...
Repeat for other elements like <link> tags, text, or anything else. You could also just have one entire template file per domain (<roundcube:include file="domain1template.html" />), but that'll be messy unless each one will be very different.
In your other thread, you asked about virtualhosts. If you do what I posted, you will also need to add your other hosts are ServerAliases.
ServerName mail.domain1.com
ServerAlias mail.domain2.com
ServerAlias mail.domain3.com
ServerAlias ...
However, you should only do all this if each domain will have SSL. Sacrificing security for whitelabeling isn't worth it in my opinion.