Explicitly define diff colors in light theme

Signed-off-by: Justin Sleep <justin@midnightmechanism.com>

res/themes/dark/css/_dark.scss

@@ -244,7 +244,7 @@ $breadcrumb-placeholder-bg-color: #272c35;
     }
 }
 
-// Fixes diff color inversion by swapping add / del colors
+// diff highlight colors
 .hljs-addition {
     background: #fdd;
 }

res/themes/light/css/_light.scss

@@ -338,3 +338,12 @@ $breadcrumb-placeholder-bg-color: #e8eef5;
     color: $accent-color;
     text-decoration: none;
 }
+
+// diff highlight colors
+.hljs-addition {
+    background: #dfd;
+}
+
+.hljs-deletion {
+    background: #fdd;
+}