tuned colors according to Light+ and Dark+ themes;

--- a/src/Tools/VSCode/extension/package.json	Sat Mar 11 12:12:55 2017 +0100
+++ b/src/Tools/VSCode/extension/package.json	Sat Mar 11 12:24:54 2017 +0100
@@ -96,6 +96,8 @@
                 "isabelle.warning_dark_color": { "type": "string", "default": "rgba(255, 140, 0, 1.0)" },
                 "isabelle.spell_checker_light_color": { "type": "string", "default": "rgba(0, 0, 255, 1.0)" },
                 "isabelle.spell_checker_dark_color": { "type": "string", "default": "rgba(86, 156, 214, 1.00)" },
+                "isabelle.main_light_color": { "type": "string", "default": "rgba(0, 0, 0, 1.00)" },
+                "isabelle.main_dark_color": { "type": "string", "default": "rgba(212, 212, 212, 1.00)" },
                 "isabelle.keyword1_light_color": { "type": "string", "default": "rgba(175, 0, 219, 1.00)" },
                 "isabelle.keyword1_dark_color": { "type": "string", "default": "rgba(197, 134, 192, 1.00)" },
                 "isabelle.keyword2_light_color": { "type": "string", "default": "rgba(9, 136, 90, 1.00)" },

--- a/src/Tools/VSCode/extension/src/decorations.ts	Sat Mar 11 12:12:55 2017 +0100
+++ b/src/Tools/VSCode/extension/src/decorations.ts	Sat Mar 11 12:24:54 2017 +0100
@@ -35,6 +35,7 @@
 ]
 
 const text_colors = [
+  "main",
   "keyword1",
   "keyword2",
   "keyword3",