Hi,
This would require CSS tweaks, but the theme layout is responsive, it always fits to the screen width. If you mean reducing the font size, you can try this CSS:
Please try adjusting zoom/scale on the OS level, you can also adjust resolution. If you use a 13-inch notebook with 120% scaling, then this is expected appearance, zooming it out may be possible via CSS customizations but I don’t recommend this because this will affect devices with 100% scaling.