A unique challenge of designing interface elements for end users in content management systems are the projects where you must create elements for the front end of sites which will be used in many unknown contexts. The usage of Content Management Systems has made this a necessity for anyone designing plugins. In my situation, I design for a specific system in which I work in-house. This affords me with some insight on user context. However, with the many templates available to our clients it is a very tight balance between clean and usable while keeping in mind what could be the worst case scenario. This makes you think (and test!) very hard when you design, especially when it comes to color and motion.
There are a few scenarios where the font from our CMS peeks through and a bright color would be appropriate for any site using the element (an alert system, for example). However, without the use of much color in most scenarios it is a unique design challenge in today's high contrast UI trends. Then there is the consideration of accessibility which really mucks up the waters.
Will we allow the user template to effect parts in the widget? Sometimes. Sometimes there just isn't much more we can do other than recommendations which we share with our clients who have customization needs. Other times, we stick with neutral all the way.
Anyone who has ever designed for a CMS knows the horrors which they have nightmares about. 20 word content titles in <h2> tags. Giant images that are contained by HTML only. Unexpected, awful, default colors that have !important declared by their webmaster.
It is a challenge I take on constantly, both in our web app and native mobile apps where we allow client branding customization. It is not easy but just like any other design challenge, if you plan well you can account for all of the above and then some. This is just another wonderful thing I love about in-house. I feel like I have the resources I need to do that pretty darn effectively.