I love the elementary default theme so I do not want to change it. Therefore, the Elementary tweaks is worthless for what I am looking forward to do... I wanted to change some specific icons and I discovered on my own how to replace some of them. I opened Files manager as an Admnistrator, went to /usr/share/applications/, opened the app file with Scratch and changed the icon location.
Fair enough. But I wanted to go further.
I was then able to change the Telegram icon in the Wingpanel : we have to go in home/user/.TelegramDesktop/tdata/ticons/.
And now I want to change the last few things in the default theme. First, I would like to change the wifi and battery icons. But I don't quite know where the wingpanels icons are. I would say they are around here : /usr/share/icons/elementary/panel/. But this is where my guesses end. Worse is yet to replace them correctly...
Furthermore, I wanted to put the windows controls (that is, close-minimize-maximize) as in the Fluch theme : https://lh5.googleusercontent.com/-B-Irmuz7Vq4/VW8WpS8AuPI/AAAAAAAAAhM/ABKurNrfbCI/w1038-h584-no/Bildschirmfoto%2Bvom%2B2015-06-03%2B16%253A48%253A05.png (from this theme : https://github.com/Keilbritschn/elementaryOS-Fluch-theme/) Turns out that I was successful. Well... sort of... I went here /usr/share/themes/elementary/gtk-3.0/, added the icons of the "windows control" folder of the Fluch theme, and replaced the apps.css file. The windows controls were changed but 1) only in the Files manager in Admnistrator mode and 2) the whole system became unstable.