... it's just wrong to change that file as you will loose that change with every update.
Bad idea.
Finding the right selector is easy as 1, 2, 3 ...
Just use CSS specificity for the basic html elements.
As I said before, I am not a developer like you guys and I am aware that updates could cause some headache. However, this can be prevented by backing up the modified less file.
Ok!
I am thinking about using another font just for headings. What do I need then to specify in the customized css file to get rid just of the problem described above, if I stick to the default font "Ubuntu".