Request: Remove hard-coded margins, padding

[tobydriscoll]

It looks as though Format.jl sets the CSS border, margins, and padding of a pre.hljl selector using hard-coded values. In my testing, a theme cannot override these settings. This complicates downstream usage. Most saliently, the caller cannot set them in the pre selector for itself and have those values respected across a variety of containers. (See https://github.com/mpastell/Weave.jl/pull/172#issuecomment-462839397.)

Options would be to remove these values from the pre.hljl selector in Format.jl, or make the values changeable via the theme.

[mortenpi]

I agree that hard-coded values in the CSS are not ideal. If nothing else, we could make it configurable, so that you can override the values or have a way of opting out of creating the pre.hljl block here:

https://github.com/JuliaDocs/Highlights.jl/blob/1f498a64013582b99ff2593c1c6959b1ef5a3fd6/src/Format.jl#L96-L105