The color scheme itself doesn't modify the UI apart from colors, the screenshot you linked from the oxocarbon repo is in context of the nyoom.nvim distro (see bottom left).

I did a quick try of oxocarbon overriding the theme from a base kickstart.nvim and it looks like below:

Its looks completely different from base but it is just careful color selection for foreground + background colors for titles, and setting the borders to off-black to give a borderless but padded look. eg vim.api.nvim_set_hl(0, "TelescopeResultsTitle", {fg = oxocarbon.blend, bg = oxocarbon.blend}) https://github.com/nyoom-engineering/oxocarbon.nvim/blob/main/lua/oxocarbon/init.lua#L282-L292