fix(highlights): if color_icons is false set to NONE

This was previously set to "fg" which only works if the user actually
has set a normal background which most people do except for transparency
users.

fixes #702

lua/bufferline/highlights.lua

@@ -154,7 +154,7 @@ function M.set_icon_highlight(state, hls, base_hl)
   })[state]
 
   local color_icons = config.options.color_icons
-  local color = not color_icons and "fg" or nil
+  local color = not color_icons and "NONE"
   local hl_colors = vim.tbl_extend("force", parent, {
     fg = color or colors.get_color({ name = base_hl, attribute = "fg" }),
     ctermfg = color or colors.get_color({ name = base_hl, attribute = "fg", cterm = true }),