In last update, Apple introduced new specification of tabs to Safari 4.
Though I wanted to modify it, Xcode 2.5 in Mac OS X 10.4 (aka Tiger) could not open Browser.nib.
A brief struggle told me a good solution that I will tell you here.

  1) First, download Pierre Chatel's Unifier to remove metal appearance.
  2) Second, change files in /Safari.app/Contents/Resources/ as below.

Then you can enjoy the favorite appearance in brandnew Safari 4.

1. Face of active windows
AW ActiveTabFill.png, AW ActiveTabLeftCap.png, AW ActiveTabRightCap.png,
AW InactiveTabBG.png, AW InactiveTabLeftCap.png, AW InactiveTabRightCap.png
AW ActiveTabClose.png, AW ActiveTabClosePressed.png, AW ActiveTabCloseRollover.png
AW InactiveTabClose.png, AW InactiveTabClosePressed.png, AW InactiveTabCloseRollover.png
AW AddTabButtonRollover Pressed.png, AW AddTabButtonRollover.png,
AW AddTabButtonRolloverBG Pressed.png, AW AddTabButtonRolloverBG.png

2. Face of inactive windows
IW ActiveTabFill.png, IW ActiveTabLeftCap.png, IW ActiveTabRightCap.png,
IW InactiveTabBG.png, IW InactiveTabLeftCap.png, IW InactiveTabRightCap.png
IW ActiveTabClose.png, IW ActiveTabClosePressed.png, IW ActiveTabCloseRollover.png
IW InactiveTabClose.png, IW InactiveTabClosePressed.png, IW InactiveTabCloseRollover.png

3. Face of loading indicator
LoadingCornerFill.png, LoadingCornerFillPush.png, LoadingCornerLeftCap.png,
LoadingCornerLeftCapPush.png, LoadingCornerRightCap.png, LoadingCornerRightCapPush.png,
LoadingCornerWhiteFill.png, LoadingCornerWhiteFillPush.png, LoadingCornerWhiteLeftCap.png,
LoadingCornerWhiteLeftCapPush.png, LoadingCornerWhiteRightCap.png,
LoadingCornerWhiteRightCapPush.png, URLBarStopButtonWhite.png
InactiveLoadingCornerFill.png, InactiveLoadingCornerLeftCap.png, InactiveLoadingCornerRightCap.png,
InactiveLoadingCornerWhiteFill.png, InactiveLoadingCornerWhiteLeftCap.png, InactiveLoadingCornerWhiteRightCap.png
/Safari.app/Contents/Resources/English.lproj/Localizable.strings

4. Face of bookmarks bar
Tiger AW ToolbarGradient.png, Tiger IW ToolbarGradient.png

5. Face of loading indicator of Safari 4.1.x
Instead of LoadingCornerxxx.png or InactiveLoadingCornerxxx.png…
URLFieldCapLeft.png, URLFieldCapLeftInactive.png, URLFieldCapLeftInactiveProgress.png,
URLFieldCapLeftProgress.png, URLFieldCapLeftRoundedInactiveProgress.png, URLFieldCapLeftRoundedProgress.png,
URLFieldCapRightInactiveProgress.png, URLFieldCapRightProgress.png, URLFieldMiddleInactiveProgress.png,
URLFieldMiddleProgress.png