I have downloaded 10.0 alpha, maybe in older versions it is still ui.set_theme - init.lua is executed at start and all "buffers" (whatever it means) are affected.
Please try both versions, eg:
ui.set_theme('dark', {font = 'Liberation Mono', fontsize = 32})
buffer.set_theme('light', {font = 'Courier', fontsize = 24})
One of them should work.
