The nodes wire was using 'TH_HEADER' flag to get its color and thus would be in sync with the header. Now make it so it uses its 'own' flag (actually 'TH_SYNTAX_R', the only TH_SYNTAX_* which wasn't yet used by the nodes).
Also expose the setting to the user so it can be themified.
This fixes T42209
Reviewers: lukastoenne
Reviewed By: lukastoenne
Maniphest Tasks: T42209
Differential Revision: https://developer.blender.org/D827