Fixed colour for given branch name in graph view

Currently commit graph seems to randomly choose the colours for branches. To make the graph easier to read, I would like to be able to configure the colours for given branches. For example, always use cyan for "master" and yellow for "develop", and never use those colours for other branches, even if "master" and "develop" are not visible.

In addition a configuration which would use a bolder line for the current branch might make the graph easier to read.