fix background color when viewing source code in documentation. (#109)

Author: Vizonex

aiohttp_theme/theme.conf

@@ -109,7 +109,7 @@ narrow_sidebar_fg = #FFF
 narrow_sidebar_link =
 font_size = 17px
 caption_font_size = inherit
-viewcode_target_bg = #ffd
+viewcode_target_bg = #424750
 code_bg = #ecf0f3
 code_text = #222
 code_hover = #EEE