Add the following lines to your .vimrc for Vim and init.vim for Neovim:
Vimscript (for Vim and Neovim with init.vim)
highlight Normal guibg=none
highlight NonText guibg=none
highlight Normal ctermbg=none
highlight NonText ctermbg=none
Lua (for Neovim with init.lua)
vim.cmd [[
highlight Normal guibg=none
highlight NonText guibg=none
highlight Normal ctermbg=none
highlight NonText ctermbg=none
]]
This should make your Vim / Neovim background transparent.