I have managed to resolve the issue.
After looking through the code several times I realised the problem was mis-matched case between the page-source and the .css file.
To remedy this situation I changed each of the following class entries in the '../DesktopModules/Forum/Themes/Default/default.css' file to a lowercase 'L' to match the source code:
.Forum_RowHighLight1 > .Forum_RowHighlight1
.Forum_RowHighLight1_Alt > .Forum_RowHighlight1_Alt
.Forum_RowHighLight2 > .Forum_RowHighlight2
.Forum_RowHighLight2_Alt > .Forum_RowHighlight2_Alt
.Forum_RowHighLight3 > .Forum_RowHighlight3
.Forum_RowHighLight3_Alt > .Forum_RowHighlight3_Alt
...
I changed those and it worked fine.
Regards, KK.