To use a custom font family, the user can use the family argument of the par() function before plotting the map. For example, use par(family = "Purisa") to use the Purisa font family.
Note that the font has to be installed on your system and recognized by R.

library(mapsf)
# Import the sample data set
mtq <- mf_get_mtq()
# population density (inhab./km2) using sf::st_area()
mtq$POPDENS <- 1e6 * mtq$POP / st_area(mtq)
# Define the font family
# !! Select a font already installed on your system !!
par(family="Purisa")
# set a theme
mf_theme("green")
# plot population density
mf_map(
  x = mtq, 
  var = "POPDENS",
  type = "choro",
  breaks = "q6",
  nbreaks = 4,
  pal = "Greens",
  border = "grey60", 
  lwd = 0.5,
  leg_val_rnd = 0,
  leg_pos = "topright", 
  leg_title = "Population Density\n(people per km2)"
) 
# layout
mf_layout(title = "Population Distribution in Martinique", 
          credits = paste0("Sources: Insee and IGN, 2018\n",
                           "mapsf ", 
                           packageVersion("mapsf")))