28bd5a7832
The theme property for group socket nodes was set to `#000000` for
default themes, which looked out of place against the other node
header colors. This patch adds colors to both the Blender Dark & Light
themes that should resemble their counterparts before 2ea3cd2188.
Colors:
Blender Dark - `#1d1d1d`
Blender Light - `#3d3d3d`
Pull Request: https://projects.blender.org/blender/blender/pulls/141011
