Fix: Skip BASE16_THEME lookup when colors are passed directly to setup()

[behcet]

Problem

When using vim.cmd.colorscheme('base16-xyz'), the colorscheme .vim file passes colors directly to setup():

  lua require('base16-colorscheme').setup({
      \ base00 = '#303446', base01 = '#292c3c', ...
      \})

However, setup() unconditionally checks BASE16_THEME and accesses M.colorschemes[vim.env.BASE16_THEME] at line 141, even when colors are already provided. This triggers the metatable __index, which calls require('colors.' .. key).

This fails when the lua/colors/ directory isn't in package.path which can happen depending on how plugin managers set up paths.

The issue is masked when running inside tmux because the vim.env.TMUX ~= nil check skips this code path entirely.

Fix

Only perform the BASE16_THEME lookup when colors is nil:

This is backwards compatible. The BASE16_THEME auto-detection only makes sense when no colors are explicitly provided.