To make certain font as the default for all the article elements of the same type, open the default format for that type in the EdAdmin4, and then select that font as the default font.
Example
You want that all body elements in every article use the Times-Roman 9pt font.
In the Formats of the EdAdmin4 (Editorial configuration > Formats), select the format, and then click Character to display the Text Font palette. In Font, select the default font.
You may also set other parameters.