improve: visibility & accessibility (#4202)

* Increase font weight for property-format

* Increase font weight for model property examples.

src/core/components/primitive-model.jsx

@@ -2,7 +2,7 @@ import React, { Component } from "react"
 import PropTypes from "prop-types"
 import { getExtensions } from "core/utils"
 
-const propStyle = { color: "#999", fontStyle: "italic" }
+const propStyle = { color: "#6b6b6b", fontStyle: "italic" }
 
 export default class Primitive extends Component {
   static propTypes = {

src/style/_variables.scss

@@ -188,7 +188,7 @@ $section-models-model-title-font-color: $gray-700 !default;
 
 $prop-type-font-color: #55a !default;
 
-$prop-format-font-color: $gray-300 !default;
+$prop-format-font-color: $gray-600 !default;
 
 // Tables