Make checkbox mark colours same as text colours

The new icon set changes the checkmark to something a lot thinner and more subtle. Making it darker compensates for that a bit.

resources/themes/cura-light/theme.json

@@ -350,7 +350,7 @@
         "checkbox_hover": [255, 255, 255, 255],
         "checkbox_border": [199, 199, 199, 255],
         "checkbox_border_hover": [50, 130, 255, 255],
-        "checkbox_mark": [50, 130, 255, 255],
+        "checkbox_mark": [35, 35, 35, 255],
         "checkbox_disabled": [223, 223, 223, 255],
         "checkbox_text": [35, 35, 35, 255],