I have a design that currently looks something like this:
I feel that there should be some sort of visual split between the main content of the post and the options below it (the up and down arrows, etc.).
Note that the design isn’t a block-styled design (like Facebook) where each post gets its own block. In this case, the next and previous post is immediately under the bottom border.
How can I make a visual distinction between the post options and the post content?
Personally, I agree with DA that whitespace is most useful here, and maybe just increasing that will be enough.
A few other possible options:
Use a thicker line between the posts, and then use a thin line like the existing one to separate the post content from options.
Use a background colour such as grey, or the other colour in your scheme, on the post options. If you chose a proper colour (not grey) I’d recommend “washing it out” as much as possible beforehand, because absolutely any colour is likely to create a strong contrast against white.
Use a simple faded background pattern, such as faint diagonal lines. It depends on your overall design, but a simple pattern could work.
It’s pretty difficult to offer suggestions when we can’t see the overall context we’re suggesting it for (the whole page), so I’ve given options for you try.