All HTML files of the help documentation use a style sheet. The name of this file is style1.css and it is located in the folder ..../Trinc-Prolog/Program/Doc/html.
It is not necessary to adapt that file with a text editor because there is a dialog window in Trinc-Prolog that can alter that file so that all HTML files will automatically change. From the main menu select 'Help | Help Properties...' and the following dialog window appears:
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The three controls at left top determine the font to use for the normal text and the three controls at the right top determine the text inside the tables. The remaining controls influence the look of the popup help windows that display information about the built-in predicates of Trinc-Prolog.