Add Lightning theme (#139)

Uses colors from the [Salesforce Lightning Design System](https://www.lightningdesignsystem.com)

data/themes.yml

@@ -178,3 +178,4 @@
 - { colors: '#1B1C5B,#1B1C5B,#8E3441,#FFFFFF,#914751,#FFFFFF,#2ACB46,#F5804F', name: 'The Incomparable'}
 - { colors: '#333333,#1E2320,#709080,#FFFFFF,#1E2320,#FFFFFF,#F0DFAF,#CC9393', name: 'Dark Zenburn'}
 - { colors: '#ebe5da,#d6b498,#c2804a,#ffffff,#b09582,#000000,#009917,#d95b5b', name: 'Latte Time'}
+- { colors: '#f4f6f9,#e0e5ee,#d8edff,#0070d2,#e0e5ee,#16325c,#4bca81,#c23934', name: 'Lightning'}