In the source it calls to csshandler.ashx in site 3 only.
Having the fact that it does not work without the skin in site 1, I used SysInternals Process Monitor when loading a page from site 3 after iisreset. Here is what I got:
219676 18:13:51,1977226 w3wp.exe 5900 QueryOpen F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS CreationTime: 12.03.2009 15:03:58, LastAccessTime: 12.03.2009 15:03:58, LastWriteTime: 12.03.2009 15:03:58, ChangeTime: 12.03.2009 15:03:58, AllocationSize: 0, EndOfFile: 0, FileAttributes: A
219677 18:13:51,1982575 w3wp.exe 5900 QueryOpen F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS CreationTime: 12.03.2009 15:03:58, LastAccessTime: 12.03.2009 15:03:58, LastWriteTime: 12.03.2009 15:03:58, ChangeTime: 12.03.2009 15:03:58, AllocationSize: 0, EndOfFile: 0, FileAttributes: A
219678 18:13:51,1985657 w3wp.exe 5900 QueryOpen F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles SUCCESS CreationTime: 12.03.2009 14:58:32, LastAccessTime: 12.03.2009 18:13:48, LastWriteTime: 12.03.2009 15:14:15, ChangeTime: 12.03.2009 15:14:15, AllocationSize: 0, EndOfFile: 0, FileAttributes: D
219679 18:13:51,1988889 w3wp.exe 5900 CreateFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles SUCCESS Desired Access: Read Data/List Directory, Synchronize, Disposition: Open, Options: Directory, Synchronous IO Non-Alert, Attributes: n/a, ShareMode: Read, Write, Delete, AllocationSize: n/a, OpenResult: Opened
219680 18:13:51,1989251 w3wp.exe 5900 QueryDirectory F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS Filter: sitesettingscachedependecy.config, 1: sitesettingscachedependecy.config
219681 18:13:51,1989688 w3wp.exe 5900 CloseFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles SUCCESS
219683 18:13:51,1992920 w3wp.exe 5900 CreateFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS Desired Access: Read Control, Disposition: Open, Options: Open Reparse Point, Attributes: n/a, ShareMode: Read, Write, Delete, AllocationSize: n/a, OpenResult: Opened
219684 18:13:51,1993280 w3wp.exe 5900 QuerySecurityFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config BUFFER OVERFLOW Information: Owner, Group, DACL
219685 18:13:51,1993442 w3wp.exe 5900 CloseFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS
219687 18:13:51,1996637 w3wp.exe 5900 CreateFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS Desired Access: Read Control, Disposition: Open, Options: Open Reparse Point, Attributes: n/a, ShareMode: Read, Write, Delete, AllocationSize: n/a, OpenResult: Opened
219688 18:13:51,1996926 w3wp.exe 5900 QuerySecurityFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS Information: Owner, Group, DACL
219689 18:13:51,1997077 w3wp.exe 5900 CloseFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles\sitesettingscachedependecy.config SUCCESS
219691 18:13:51,2000960 w3wp.exe 5900 CreateFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles SUCCESS Desired Access: Read Data/List Directory, Read Attributes, Synchronize, Disposition: Open, Options: , Attributes: n/a, ShareMode: Read, Write, Delete, AllocationSize: n/a, OpenResult: Opened
219692 18:13:51,2003839 w3wp.exe 5900 CreateFile F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles IS DIRECTORY Desired Access: Read Data/List Directory, Synchronize, Disposition: Open, Options: Synchronous IO Non-Alert, Non-Directory File, Complete If Oplocked, Attributes: n/a, ShareMode: Read, AllocationSize: n/a
219693 18:13:51,2004368 w3wp.exe 5900 NotifyChangeDirectory F:\Billing\mojoPortal SVN\Web\Data\Sites\1\systemfiles Filter: FILE_NOTIFY_CHANGE_FILE_NAME, FILE_NOTIFY_CHANGE_DIR_NAME, FILE_NOTIFY_CHANGE_SIZE, FILE_NOTIFY_CHANGE_LAST_WRITE, FILE_NOTIFY_CHANGE_CREATION, FILE_NOTIFY_CHANGE_SECURITY
219700 18:13:51,4523798 w3wp.exe 5900 QueryOpen F:\Billing\mojoPortal SVN\Web\Data\Sites\1\skins\starlink-manager-jsavard-subblue\style.config PATH NOT FOUND
So it calls to 2 files in site 1, including one from the skin!