I changed the weight 100 to weight 300 in CSS, as I thought that perhaps the font that I have chosen (‘Open Sans’, sans-serif) doesn’t recognize value “100”. Unfortunately, it didn’t change anything, h2, h3, h5 are still too bold (except where purposefully a bold tag is inserted) – so this wasn’t the fault..
I managed to fix the buttons thou, they look ok now when viewing the website while logged out from admin panel.