For example using set_theme!, you can also pass the fonts setting via with_theme or to the Figure constructor.
MT = Makie.MathTeXEngine
mt_fonts_dir = joinpath(dirname(pathof(MT)), "..", "assets", "fonts", "NewComputerModern")
set_theme!(fonts = (
regular = joinpath(mt_fonts_dir, "NewCM10-Regular.otf"),
bold = joinpath(mt_fonts_dir, "NewCM10-Bold.otf")
))
x = LinRange(-pi,pi,100)
f, ax = lines(x,cos.(x),label=L"The function $y(x)=\sin(x)$")
axislegend()
ax.xlabel = L"x"
ax.ylabel = L"y"
ax.title = "This is a test title"
f
