config: change color format to 0xRRGGBBAA

The current format of #RRGGBBAA is problematic as # starts a comment
in POSIX compliant shells, requiring escaping/quoting and increasing
complexity.

This is a breaking change.

example/init

@@ -140,6 +140,11 @@ do
     riverctl map $mode None XF86MonBrightnessDown spawn 'light -U 5'
 done
 
+# Set background and border color
+riverctl background-color 0x002b36
+riverctl border-color-focused 0x93a1a1
+riverctl border-color-unfocused 0x586e75
+
 # Set repeat rate
 riverctl set-repeat 50 300