Add support for colourblind themes

Workaround

Not at the moment. Examples for the types of options available for GitHub that may be useful for comparison can be found under https://github.com/settings/appearance

Problem

Users with colourblindness struggle to distinguish between the red/green colour palette used to denote whether runs and stacks have succeeded or failed as well as the proposed changes (e.g. additions, deletions, changes etc) in the currently available set of theme options