EDIT (30/12/16): I have unpublished the extension from Chrome store and retaining this post and the GitHub repo. Stylish is a better extension and offers more features. This extension was a good learning experience for me on how to develop Chrome extension.
Site Highlighter is a Chrome extension I developed to do only one task: change the background colour of an element or a tag. The element could be selected by classname or id. I will give you an example. CRM 2013 doesn’t support themes. But if you just want to differentiate the various environments (DEV/TEST/PROD) by the navigation bar colour, it is not possible without changing the css using the the CRM web application. This is easy to do in OnPremise, but is technically an unsupported change.
In this instance you can use my extension to change the navigation bar to a different colour. The ID you can to match is navBar. The screenshot below shows you the config menu of the extension.
After you set this up the extension will highlight the element by id navbar, with green colour. The source code for the extension is available at https://github.com/rajyraman/Site-Highlighter. You can install the extension from https://chrome.google.com/webstore/detail/site-highlighter/hembcpepmfkaigjalcajpjjbeimbhnnk
This is my first extension and I worked on it only for three days, including constant ALT+TABing the Chrome Extension developer documentation, and so it may be a little rough around the edges.
Request to people with expertise in Chrome extension Development/UX & Users: If you can give your thoughts on things that I am not doing correctly, it will be helpful for me to make improvements. Thank you.