Changing Normal highlight color sets background back to light. How to prevent?

[trusktr]

When I try

highlight Normal ctermfg=250 ctermbg=233 guifg=#c5c8c6 guibg=#1d1f21

this seems to set background to light and completely changes what I intended to change.

How do I prevent this? I just want to make Hybrid's background slightly darker (from 234 to 233) in my vimrc, and stay in background=dark mode.