One useful trick for customizing webwork messages is to take advantage of the localization framework that is now available. For example, our college algebra course has a special 'translation' file which customizes many of the displayed strings, including the reduced credit message, and in that course the "Homework Sets" link in webwork now says "Homework and Quizzes" and we use the word "quiz" in place of "test" or "gateway."
To do this you could just copy en.po under a new name (e.g. dl.po) and start editing the msgstr's that you want to change. After restarting webwork you should then see the option "dl" under "Languages" in "Course Configuration."
And, regarding changing the color, yes, the way to do this is to adapt an existing theme.
Hope this helps
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"carrot vs stick" for Reduced Credit
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