Add descriptions to light theme

Co-authored-by: Sam Cunliffe <samcunliffe@users.noreply.github.com>

Author: dstansby

src/napari_matplotlib/styles/light.mplstyle

@@ -1,8 +1,8 @@
 # Light-theme napari colour scheme for matplotlib plots
 
-# text ():       #3b3a39
-# foreground (): #d6d0ce
-# background (): #efebe9
+# text (very dark grey - almost black): #3b3a39
+# foreground (mid grey):                #d6d0ce
+# background (brownish beige):          #efebe9
 
 figure.facecolor    : none
 axes.labelcolor     : 3b3a39