This is not a bug, but a design choice.

It is also much more usable and color-compatible between a lot of different colorschemes (dark themes first of all).

I'm sure you'll like more the new style after few days of usage (but with a nicer theme than the one you posted :) try to use the default Clearlooks theme with the correct x,y thickness)

Closing and marking as NOTABUG

Thanks for the clarification.  IMO the gradient style tabs look nicer.  Maybe you could toss in a tab style option and have both styles available?

  engine "clearlooks" 
  {
    #scrollbar_color   = "#76acde"
--> tabstyle          = 1 # 0 = line, 1 = gradient
    colorize_scrollbar = TRUE
    menubarstyle      = 2 # 0 = flat, 1 = sunken, 2 = flat gradient
    toolbarstyle      = 1 # 0 = flat, 1 = enable effects
    animation         = FALSE
    style             = GLOSSY
  }
}

Keep up the good work!

With gradient - Gummy was unique - has personality - with flat color look more a simple worker which only know to do his job OK ...but that's all.

Tabs with gradients look refined. And if Italians do it better - well for me that was the difference, that was the "big" drop of refinement and personality. 

This was attractive for Clearlooks but because I liked this too much and I lost it ;) I go now with Aurora ( exact for the same reason - they had Gradients on tabs also on scrollbars bg. and in some form widgets - this  give them some deep and "Pro" look).

gradients on tabs were one of my biggest mistakes, since they were bad looking and looked out of place with the rest of the theme.

I don't know if your *opinion* is just a rant but I guess so.
Remember that the majority of people thinks like me so words like *pro* *deep* have no value...

In cany case feel free to use what you want, Aurora looks out of place with a mix of glassy and gradient widgets, it contains a lot of hacks in the code, it is much slower than every engine, even slower than a pixmap theme...

But don't come there and rant :)

Cheers