Add variables to allow setting different light/dark colorschemes

Previously this already was accomplishable with the User callbacks
LumenLight and LumenDark, but these variables serve as a shortcut for
that.

Author: vimpostor

README.md

@@ -48,6 +48,8 @@ au User LumenLight echom 'Entered light mode'
 au User LumenDark echom 'Entered dark mode'
 ```
 
+Note that for the common usecase of switching the colorscheme, there are the `g:lumen_light_colorscheme` and `g:lumen_dark_colorscheme` variables.
+
 ## What are some good light colorschemes?
 
 There are not many colorschemes that work well both in light and dark mode.

autoload/lumen.vim

@@ -20,7 +20,17 @@ func lumen#init()
 	augroup END
 endfunc
 
+func lumen#apply_colorscheme(mode)
+	" apply colorscheme from g:lumen_light_colorscheme/g:lumen_dark_colorscheme
+	let v = get(g:, printf("lumen_%s_colorscheme", a:mode), '')
+	if len(v) && get(g:, 'colors_name', '') != v
+		exe printf("colorscheme %s", v)
+	endif
+endfunc
+
 func lumen#light_hook()
+	call lumen#apply_colorscheme('light')
+
 	if s:background == 'light' && &background == s:background
 		return
 	endif
@@ -33,6 +43,8 @@ func lumen#light_hook()
 endfunc
 
 func lumen#dark_hook()
+	call lumen#apply_colorscheme('dark')
+
 	if s:background == 'dark' && &background == s:background
 		return
 	endif

doc/lumen.txt

@@ -54,6 +54,22 @@ CONFIGURATION						*lumen-configure*
 		will no longer influence vim's internal detection mechanism at
 		startup.
 
+	g:lumen_light_colorscheme			*g:lumen_light_colorscheme*
+		This variable defines the colorscheme to change to (if any),
+		when the system switches to light mode. For a more generic
+		callback, consult |User_LumenLight|.
+		For example `let g:lumen_light_colorscheme = 'retrobox'` is
+		equivalent to `autocmd User LumenLight colorscheme retrobox`,
+		also see |g:lumen_dark_colorscheme|.
+
+	g:lumen_dark_colorscheme			*g:lumen_dark_colorscheme*
+		This variable defines the colorscheme to change to (if any),
+		when the system switches to dark mode. For a more generic
+		callback, consult |User_LumenDark|.
+		For example `let g:lumen_dark_colorscheme = 'retrobox'` is
+		equivalent to `autocmd User LumenDark colorscheme retrobox`,
+		also see |g:lumen_light_colorscheme|.
+
 ------------------------------------------------------------------------------
 AUTOCOMMANDS						*lumen-autocommands*