Fix a bug about termguicolors option

We can not detect whether nvim-qt is loaded when sourcing `init.vim`.
If termguicolors option is not set, we use a better color (badwolf in this case).
Gruvbox8 is terrible when termguicolors is not set.

ui.vim

@@ -2,15 +2,13 @@
 "{{ General settings about colors
 " Enable true colors support (Do not use this option if your terminal does not
 " support true colors! For a comprehensive list of terminals supporting true
-" colors, see https://github.com/termstandard/colors and https://bit.ly/2InF97t.
-if exists("&termguicolors")
-    if $TERM == "xterm-256color" || exists('g:GuiLoaded')
-        set termguicolors
-    else
-        set notermguicolors
-    endif
+" colors, see https://github.com/termstandard/colors and
+" https://bit.ly/2InF97t)
+if $TERM == "xterm-256color"
+    set termguicolors
+else
+    set notermguicolors
 endif
-
 " Use dark background
 set background=dark
 "}}
@@ -19,16 +17,27 @@ set background=dark
 """"""""""""""""""""""""""""gruvbox settings"""""""""""""""""""""""""""
 " We should check if theme exists before using it, otherwise you will get
 " error message when starting Nvim
-if utils#HasColorscheme('gruvbox8')
-    " Italic options should be put before colorscheme setting,
-    " see https://goo.gl/8nXhcp
-    let g:gruvbox_italics=1
-    let g:gruvbox_italicize_strings=1
-    let g:gruvbox_filetype_hi_groups = 0
-    let g:gruvbox_plugin_hi_groups = 0
-    colorscheme gruvbox8_hard
+if &termguicolors
+    if utils#HasColorscheme('gruvbox8')
+        " Italic options should be put before colorscheme setting,
+        " see https://goo.gl/8nXhcp
+        let g:gruvbox_italics=1
+        let g:gruvbox_italicize_strings=1
+        let g:gruvbox_filetype_hi_groups = 0
+        let g:gruvbox_plugin_hi_groups = 0
+        colorscheme gruvbox8_hard
+    else
+        colorscheme desert
+    endif
 else
-    colorscheme desert
+    if utils#HasColorscheme('badwolf')
+        let g:badwolf_darkgutter = 0
+        " Make the tab line lighter than the background.
+        let g:badwolf_tabline = 2
+        colorscheme badwolf
+    else
+        colorscheme desert
+    endif
 endif
 
 """"""""""""""""""""""""""" deus settings"""""""""""""""""""""""""""""""""
@@ -49,11 +58,5 @@ endif
 " " theme_style can be 'default', 'dark' or 'palenight'
 " let g:material_theme_style = 'dark'
 " colorscheme material
-
-"""""""""""""""""""""""""""badwolf settings""""""""""""""""""""""""""
-" let g:badwolf_darkgutter = 0
-" " Make the tab line lighter than the background.
-" let g:badwolf_tabline = 2
-" colorscheme badwolf
 "}}
 "}