[size=150][color=#0040FF]GOT IT!!![/color][/size] 
Here’s how to do it!
1: Download the source for Urho3D and extract it. In my case iI get the directory “Urho3D-1.32”…
2: navigate to “Urho3D-1.32/Docs” and you will see a file “Doxyfile.in” – Open this file in a text editor ( Gedit for me partially beacuse it allows you to enable displaying line numbers )
3: goto line #914 HTML_STYLESHEET =
4: Type “main-min.css” next to it ( no quotes or semi-colons or anything afterwards ) to get this:
| 913 |
| 914 | HTML_STYLESHEET = main-min.css
| 915 |
** SAVE IT **
5: To make sure Doxygen can find the “main-min.css” stylesheet, we need to download it and here’s how:
6-A: Goto the Urho3D “About” page: http://urho3d.github.io/about.html and then save it as COMPLETE HTML… in FireFox I used “File > Save Page As” ( note: + works too ) and make sure that you are saving as “Webpage, Complete” which will save all the files that the webpage links to including "main-min.css
6-B: or i suppose you could just download the stylesheet directly http://urho3d.github.io/main-min.css 
7: Be sure to copy that “main-min.css” page into the same folder as the “Doxygen.in” file --> "Urho3D-1.32/Docs"
8: open a terminal window and navigate to the directory where the Doxygen.in and the main-min.css files are “Urho3D-1.32/Docs”
9: type:
and when it gets done then it will generate a folder “html” right there in that “Urho3D-1.32/Docs” folder and then everything works beautifully including classes
10: Enjoy 
NOTE: I looked at the main-min.css file and there are URL imports in there to import files from the web such as google and github. I didn’t look too deeply as my text editor kept getting overloaded ( cpu-wise ) by the file. I did all of this on my offline PC after transfering the main-min.css file and the Urho3D source over there so there was no internet to connect to. Your results may vary ( or look nicer than mine ) but still 
Many thanks for your hints at where to look guys!