You must have the developer tools installed to do this yourself.
Right-click or control-click on the Safari package.
Click "Show Package Contents"
In the window that pops up, navigate to "Contents/Resources/YourLanguage.lproj/"
where your language will be whatever language you use for the System. Ex: English.lproj
Double click on the Browser.nib to open it in Interface Builder. If the browser window isn't already open in Interface Builder, double click the "Window" icon shown in the Browser.nib window.
Select Show Info from the tools menu.
Select the Attributes item from the drop down menu.
Uncheck Textured Window.
Save the nib file.
Enjoy your new non-metalic browser
If you don't have the developer tools installed, download the file I've attached and decompress it. Follow the instructions above and replace the Browser.nib file contained in the package with the one you downloaded.
This file attached is only valid for the beta version of Safari because I'm sure Apple will change it for the next release.
Right-click or control-click on the Safari package.
Click "Show Package Contents"
In the window that pops up, navigate to "Contents/Resources/YourLanguage.lproj/"
where your language will be whatever language you use for the System. Ex: English.lproj
Double click on the Browser.nib to open it in Interface Builder. If the browser window isn't already open in Interface Builder, double click the "Window" icon shown in the Browser.nib window.
Select Show Info from the tools menu.
Select the Attributes item from the drop down menu.
Uncheck Textured Window.
Save the nib file.
Enjoy your new non-metalic browser

If you don't have the developer tools installed, download the file I've attached and decompress it. Follow the instructions above and replace the Browser.nib file contained in the package with the one you downloaded.
This file attached is only valid for the beta version of Safari because I'm sure Apple will change it for the next release.