I tried to find any css files like those in Xfce or Gnome to modify the desktop icons font style but I could not able to find any thing in
~/.local/share/plasma/look-and-feel for the current active desktop theme.
Thanks to DK Bose, the file he regarded in the comments
~/.config/kdeglobals leads me to answer the question with some limitations.
Open the file
~/.config/kdeglobals using kate and search for the following block of code:
[General] BrowserApplication[$e]=firefox.desktop ColorScheme=Card Name=Card XftAntialias=true dbfile=/home/saidbakr/.mozilla/firefox/3tzvfegj.default/places.sqlite fixed=Hack,9,-1,5,50,0,0,0,0,0,Regular font=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular menuFont=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular shadeSortColumn=true smallestReadableFont=Noto Sans,8,-1,5,50,0,0,0,0,0,Regular toolBarFont=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular widgetStyle=Breeze
Regular in the line
font=Noto Sans,10,-1,5,50,0,0,0,0,0,Regular by
By this way the desktop icons font will be in bold.
Not only desktop icons fonts will be affected, almost all folders views in Dolphin will have bold icons font.