Set border_main for darktheme to be darker

CURA-8975

resources/themes/cura-dark/theme.json

@@ -13,7 +13,7 @@
 
         "accent_1": [25, 110, 240, 255],
         "accent_2": [16, 70, 156, 255],
-        "border_main": [212, 212, 212, 255],
+        "border_main": [118, 118, 119, 255],
         "border_accent_1": [255, 255, 255, 255],
         "border_accent_2": [16, 70, 156, 255],
         "border_field": [57, 57, 58, 255],