Rated 3 out of 5 stars

yazan: GVP tarih: Nisan 26, 2017 ·

After trying this in SeaMonkey 2.46 (Mozilla x86/x64 build) on Ubuntu 14.04, I can say that the author of this theme is correct to say that it may not look right in Linux. I despair of finding a theme that gives me the classic Netscape/SeaMonkey look of the 1990s on my platform.

This review is for a previous version of the add-on (0.1.2). 

yazan: Exalm (Geliştirici) tarih: Nisan 26, 2017 ·

As said in description, it does look good only with classic-style GTK themes. And only with GTK2, not with GTK3 (there's not much I can do about this).
Try Raleigh, or similar GTK themes.
If you want a theme that looks good with any GTK2 theme, try GNOMErunner.