You didn't mention which version of UE that you are using. This would help us to help you.
Although, if you are editing the .uew file to change colors, I can fairly safely assume that you are using at least version 15 of UE, or version 9.1 of UES.
I have done a lot of work with this, and have discovered that if there is some sort of error in the color information in the file, it will indeed reset to the default colors every time. However, this is not the ONLY reason for this to happen. In order to manually edit the colors in several of the versions of UE, you must FIRST edit the colors from within the program (such as editing at least one color in the "scheme" colors.) The main background color of "text" is actually under "View -> Set Colors" where you want to configure some of these colors.
It can be quite confusing, very tedious, and rather tiresome attempting to map out colors in UE.
It can also be very difficult to manually edit this file, and make sure there are no errors.
This is why I created a program to assist you with it. See the topic: Color Scheme Management Tool
. You may want to consider downloading the tool to update the colors for your wordfiles. Also, there are well over 1000 themes available for download. The list of themes is also linked in that first post of the topic. There is the standard desktop version of the program and a PortableApps.com version, depending on your needs. It is still in active development and is currently having new features worked on to make editing/creating wordfiles even easier fairly soon.