One of the reasons why I don't like Ubuntu's default theme Ambiance is the unacceptable inconsistency between GTK2 and GTK3 toolbars as seen in LibreOffice and Chromium/Chrome web browser.
This is not the case in the Greybird theme, for example, which proves that it is possible.
I would appreciate if someone can help tweak this.
Note: a similar problem with menus (inc. ComboBox menus) has been addressed here: Consistency of GTK2 menus with GTK3
The two screenshots below show how Greybird and Ambiance themes display applications like (from top to bottom) Chrome, LibreOffice and KKEdit (with an "undefined" toolbar bgcolor which is inappropriate with the icon theme to be used in the case of Ambiance as opposed to consistent looks of Greybird or Faience ), unlike other apps like Nautilus and gedit, which I added at the bottom of the Ambiance screenshot.
Greybird theme:
Ambiance theme:
I've found that inserting the following lines with appropriate "color codes" (e.g. @dark_bg_color , #808080, etc.) in the section style "toolbar" might be a solution, although this might lead to labels etc. in some gtk2 apps unreadable (perhaps a solution might also be found for this?)