[update] move default value of the customCSS field to ConfigManager

Author: KazumiHARADA

browser/main/lib/ConfigManager.js

@@ -85,7 +85,7 @@ export const DEFAULT_CONFIG = {
     breaks: true,
     smartArrows: false,
     allowCustomCSS: false,
-    customCSS: '',
+    customCSS: '/* Drop Your Custom CSS Code Here */',
     sanitize: 'STRICT', // 'STRICT', 'ALLOW_STYLES', 'NONE'
     lineThroughCheckbox: true
   },

browser/main/modals/PreferencesModal/UiTab.js

@@ -883,7 +883,6 @@ class UiTab extends React.Component {
                   onChange={e => this.handleUIChange(e)}
                   ref={e => (this.customCSSCM = e)}
                   value={config.preview.customCSS}
-                  defaultValue={'/* Drop Your Custom CSS Code Here */\n'}
                   options={{
                     lineNumbers: true,
                     mode: 'css',