First I apologize if I am not using the correct terminology to explain this.

There are several themes that have transparency in the window decorations. But what I want is to continue this to remove the amount of grey I have in KDE. Is there any way to introduce transparency in the menus and in the KDE "grey" areas ?

yes if you use qtcurve widget style

in system settings->Application Appearance->Style->Widget Style is set to oxygen as default I believe

after you have installed qtcurve with whatever package management system you use, you select qtcurve in the drop down list

then click configure

you can then set transparency for windows and window decorations

I found it has trouble with GTK apps tho, but you can exclude certain apps from the transparency in the settings also

Martin