Update display for white images by adding checkered bg. Fixes #1103

Also make color variables usage consistent, and fix specificity not
allowing the file_list items to have the "move" cursor

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to this project will be documented in this file.
 * Enable linking options pages via tabbed-navigation. Will output tabbed navigation for options-pages which share the same `'tab_group'` CMB2 box property. [This snippet](https://github.com/CMB2/CMB2-Snippet-Library/blob/master/options-and-settings-pages/options-pages-with-tabs-and-submenus.php) demonstrates how to create a top-level menu options page with multiple submenu pages, each with the tabbed navigation. To specify a different tab title than the options-page title, set the `'tab_title'` CMB2 box property. See [#301](https://github.com/CMB2/CMB2/issues/301), [#627](https://github.com/CMB2/CMB2/issues/627).
 * Complete the `zh-CN` translation. Props [@uicestone](https://github.com/uicestone) ([#1089](https://github.com/CMB2/CMB2/issues/1089)).
 * Update the `nl_NL` translation. Props [@tammohaannl](https://github.com/tammohaannl) ([#1101](https://github.com/CMB2/CMB2/issues/1101)).
+* Better display for white images (e.g. logos) by using a checkered background for images. ([#1103](https://github.com/CMB2/CMB2/issues/1103))
 * Ability to disable the options autoload parameter via filter (`"cmb2_should_autoload_{$options_key}"`) or via a box parameter for `'options-page'` box registrations (`'autoload' => false,`). ([#1093](https://github.com/CMB2/CMB2/issues/1093))
 
 ## [2.3.0 - 2017-12-20](https://github.com/CMB2/CMB2/releases/tag/v2.3.0)