Edit the style of Purity III template

This was a big mess for me, however I hope that I solved it.

Firstly, you have to create your Theme, or edit the existing one in directory /less/themes/

Mine is called 1912, so the final directory becames /less/themes/1912/

Than you have to create a file called custom.less
This file kind of overrides the existing css styling.

When you add any change, than you have to save it from your joomla admin panel

The final but most important part is, when you save it, you have to press "Compile LESS" - воала