Generally, all text in the diagram elements is written in standard font (black), but you may enable the option "Highlight variables?" in the menu "View" (before version 3.32-13 it was in menu "Diagram"). This does not only highlight variable names, but also operator symbols, string and character literals, and certain keywords (also see page Syntax highlighting in the Features section, page Parser Preferences, and page Controller Aliases):

Alternatively, you may press <F4> to toggle the highlighting mode.

Note that with highlighting mode switched off, the menu item beneath it, "Show operators in C style?" (as introduced with version 3.30-11) will be disabled because it would not have any impact without highlighting mode (but it will keep its state until syntax highlighting will be switched on again).

Here is an example for the highlighting effect:

Plain diagram versus highlighted diagram (in standard and C operator mode)

You will find plenty of examples throughout this User Guide and, of course, in practical use — it's quite straightforward.

Since version 3.30-14, syntax highlighting also applies to the popup showing the element text in display mode "Switch text/comments":

Remark: Before version 3.29-06, highlighting was the major cause of drawing contention, particularly in Arranger. Since version 3.29-09, these performance problems have been eliminated. Only the first drawing of a large amount of diagrams imported by code or loaded from an arrangement file may take a few seconds, therafter the time difference to drawing without highlighting is hardly perceptible anymore.