Commit graph color doesn't follow color scheme changes

[tibirna]

Start qgit in a light color scheme. Observe the main color of the commit graph (black). With qgit running, change the global color scheme (e.g. in KDE using System Settings > Appearance > Color > Breeze Dark). Observe that the main color of the commit graph remains black (and thus barely visible in the new color scheme). With the color scheme still dark, restart qgit and notice the commit graph main color (now white). Change the color scheme back to light. Notice the commit graph color staying white, i.e. barely visible in the new color scheme.

To fix: ensure graph is redrawn on color scheme change events.