Do you see where you have
<authorization>
<deny users="?"/>
</authorization>
This means deny to anonymous users. You should try allowing your css file through then just as you allowed your login page.
<configuration>
<location path="Login.aspx">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>
<location path = "Styles/Site.css">
<system.web>
<authorization>
<allow users = "*" />
</authorization>
</system.web>
</location>
<system.web>
<authorization>
<deny users="?"/>
</authorization>
</system.web>
</configuration>