That “Bold” up there is just a preset that Boox NeoReader offers.
I took some quick snaps of the Style tab on mine - specifically the “Original” (where I can choose font, font size etc) and that has a More Settings button, and then two examples where the weight (“Bold”) is set at 0 and one where it’s set at 8, which will hopefully show some difference on the pictures. The weight can be set to quite thick but there are many gradations so it’s easy enough to pick a suitable weight.
(The lighting is quite random right now and I haven’t edited the pictures at all apart from crop and resize. And mine is the colour one so the actual look depends a fair bit on both the ambient lighting and the front light. But basically as you can see, NeoReader does offer a fair amount of settings to adjust and customise.)