To change the font size used of a trellis in all visuals that appear on this dashboard, navigate to the dashboard Style menu, select the Visual Axes and Labels sub-menu, and make adjustments in the Label Text Color selector.
In the following example, compare the two visuals; the first one uses the default color scheme in labels, and the second one uses black font color.
Note that this style option overrides the selection at the level of the individual visual.
To style the visuals individually, de-select the Override visual style settings option, and specify the corresponding setting in each visual.