feat: support themes other than defaults

Try to activate a theme if not presented in defaults. Accordingly,
add the option `ui.colors` to provide the missing `hl_theme*` in
the custom theme.

lua/core/default_config.lua

@@ -46,6 +46,7 @@ M.options = {
 
 M.ui = {
    hl_override = "", -- path of your file that contains highlights
+   colors = "", -- path of your file that contains colors
    italic_comments = false,
    theme = "onedark", -- default theme