Add preliminary dark theme

With thanks to Tobias Wolf for some colour suggestions.

Author: kaiw

data/meld-dark.css

@@ -0,0 +1,23 @@
+
+@import 'meld.css';
+
+@define-color insert-bg #227000;
+@define-color insert-outline shade(@insert-bg, 1.5);
+@define-color insert-text #4E9A06;
+@define-color delete-bg @theme_bg_color;
+@define-color delete-outline shade(@delete-bg, 1.5);
+@define-color delete-text #a40000;
+@define-color replace-bg #00458c;
+@define-color replace-outline shade(@replace-bg, 1.5);
+@define-color replace-text #2d6cfc;
+@define-color conflict-bg #7a2a28;
+@define-color conflict-outline shade(@conflict-bg, 1.5);
+@define-color conflict-text #ff0000;
+@define-color error-bg #fce94f;
+@define-color error-outline shade(@error-bg, 1.5);
+@define-color error-text #faad3d;
+@define-color inline-bg #3465a4;
+@define-color unknown-text #aaaaaa;
+@define-color current-line-highlight #111100;
+@define-color syncpoint-outline #bbbbbb;
+@define-color current-chunk-highlight alpha(#ffffff, 0.1);