janusz,
your lucky lately I've been searching ways how to change the taskbar icons etc.
These of course can be found in the themes > gnome-shell folder and especially the gnome-shell.css file.
I saw your print screen and know your using Zorin Core. If I'm right the theme you are using is set to ZorinBlue-Light.
If you navigate to /usr/share/themes/ and copy the ZorinBlue-Light folder to your; /home/user/.themes folder and navigate to the gnome-shell folder and open the gnome-shell.css file, so you can adjust the settings for these system icons.
Look for this line;
#panel .panel-button .system-status-icon { icon-size: 16px; padding: 5px; margin: 0 4px; }
If you change the icon-size: 'x'px to a certain value, the icons will change according to the vaule.
Save the file and hit Alt + F2 and 'r', to refresh the desktop.
Like this before;
After;