Sorry i don't refind original ... but it's simple just edit the files like in the images shown! ;)
You must have created your new images icons in the folder ui/icons

Edit File ui/Commanbar.html and just add the 3 lines following (Centrage vertical)



For the icon text, add to the ui/EnglishStrings.txt file your textid reference, according to the example, Origin=Origin (text under the icon)

here that is for a fonction "Origin"