doc: use basic theme as parent theme instead of default theme

doc/themes/mroonga/theme.conf

@@ -1,34 +1,14 @@
 [theme]
-inherit = default
+inherit = basic
 stylesheet = mroonga.css
 pygments_style = sphinx
 
 [options]
 rightsidebar = false
 stickysidebar = false
-collapsiblesidebar = false
 externalrefs = false
 
 sidebarwidth = 230
 
-footerbgcolor    = #ffffff
-footertextcolor  = #000000
-sidebarbgcolor   = #ffffff
-sidebarbtncolor  = #ffefd1
-sidebartextcolor = #000000
-sidebarlinkcolor = #992e1d
-relbarbgcolor    = #f6872e
-relbartextcolor  = #ffffff
-relbarlinkcolor  = #ffffff
-bgcolor          = #ffffff
-textcolor        = #000000
-headbgcolor      = transparent
-headtextcolor    = #992e1d
-headlinkcolor    = #f8e0ae
-linkcolor        = #e15611
-visitedlinkcolor = #f6872e
-codebgcolor      = #eeffcc
-codetextcolor    = #333333
-
 bodyfont = sans-serif
 headfont = sans-serif