It was that I missed to make some edits in the files
/usr/share/themes/Radiance/gtk-2.0/gtkrc and
/usr/share/themes/Radiance/metacity-1/metacity-theme-1.xml; which I edited to specify the colours that I wanted for the windows decorator.
(But still unable to fix the colour of tab-bar of the Chromium browser.)
Altogether I've made changes in the files:
/usr/share/themes/Radiance/gtk-2.0/gtkrc,
/usr/share/themes/Radiance/gtk-3.0/gtk.css,
/usr/share/themes/Radiance/gtk-3.0/settings.ini,
/usr/share/themes/Radiance/metacity-1/metacity-theme-1.xml, and
/usr/share/themes/Radiance/gtk-2.0/apps/chromium.rc
Here's a snapshot of my custom colour palette which includes most of the changes that I made in the colour values in the files mentioned above:
(Note: the info_fg_color and info_bg_color values shown above is not a very good match.)
This is what my Ubuntu desktop with the modified Radiance theme looks like:

Edit: "...all fixed but the Chromium Web Browser's tab bar!" [solved]
To modify ChromeGtkFrame (which I referred to as Chromium Web Browser's tab bar) colour changes had to be made in the /usr/share/themes/Radiance/gtk-2.0/apps/chromium.rc file; where I used the colour value #e2e2e2. Here's what Chromium window now looks like:

Courtesies to the helpful comments/answers from
vasa1 for getting the problem is fixed.