A mood of a typeface is the way the font is perceived by the majority of people and the associations it arises. The font mood is a pretty social construct, and is defined not only by its design, but also by the context it is used in. This concept is broadly used among UX-designers.
Depending on the mood, fonts fit different kinds of texts. Some moods are considered neutral, such as Helvetica or Times New Roman, meaning that they can be used regardless of the context. Gothic typefaces are generally considered moody and rather depressing. At the same time, you can affect the font mood by using color and size.
Font moods are chosen based on the text purpose. They can attract attention, indicate that something laidback or humorous is contained in the text, or, on the contrary, make a person feel serious about what’s written on the page.
Font moods help to establish communication with a reader even before he has started actually reading the text. These are kind of visual cues building the first impression for the reader. All in all, font moods increase the impact of the written passage, or ruin it, like in the picture below:
As far as technical documentation is concerned, the most used font moods would be neutral, formal ones. Although, of course, user guides have different purposes and end users as any other text. We can easily imagine a playful and childish font mood on an instruction for a toy, written for children.
Matching mood fonts with the text color and size is of primary importance if you do not want to leave readers puzzled and confused. Playful fonts won’t go with black, brown, grey-ish color palette, neutral fonts will look weird with each letter colored differently. Studying the way colors are matched in web design can help a lot with this task. Read this article to grasp the basics of how to create a color scheme.
When playing with font moods, one should not overlook the general rules of font matching. Using these rules combined with clear understanding of target audience can improve author-reader communication greatly.