Brief: Toggle the use of proportionately spaced (variable-pitch) fonts
in the User Interface.
Symbol: modus-themes-variable-pitch-ui (‘boolean’ type)
Possible values:
nil (default)
t
This option concerns User Interface elements that are under the direct control of Emacs. In particular: the mode line, header line, tab bar, and tab line.
The default is to use the same font as the rest of Emacs, which usually is a monospaced family.
With a non-nil value (t) apply a proportionately spaced typeface. This
is done by assigning the variable-pitch face to the relevant items.