Hi @Tom2
Blocs 3 is based on Bootstrap 4 and as a consequence the hamburger icon is now a svg file. If you want to change the size or/and color of the hamburger then you need to modify the svg data. There exists already a number of posts in the forum that provide the necessary information regarding the svg and how to modify it.
For instance: Hamburger Menu and horizontal lines changes - #3 by KBConcepts
You will need to change the view box specifics.
MDS