Feat: enable add_hl in themes

lua/base46/init.lua

@@ -46,6 +46,7 @@ end
 
 M.extend_default_hl = function(highlights)
   local polish_hl = M.get_theme_tb "polish_hl"
+  local add_hl = M.get_theme_tb "add_hl"
 
   -- polish themes
   if polish_hl then
@@ -56,6 +57,15 @@ M.extend_default_hl = function(highlights)
     end
   end
 
+  -- add new hl
+  if add_hl then
+    for key, value in pairs(add_hl) do
+      if not highlights[key] and type(value) == "table" then
+        highlights[key] = value
+      end
+    end
+  end
+
   -- transparency
   if vim.g.transparency then
     local glassy = require "base46.glassy"