Themes: Use "local"

All themes (except for xresources) were setting a global variable "theme" with
the defined settings. Prefix this with "local".

Signed-off-by: Uli Schlachter <psychon@znc.in>

themes/default/theme.lua

@@ -2,7 +2,7 @@
 -- Default awesome theme --
 ---------------------------
 
-theme = {}
+local theme = {}
 
 theme.font          = "sans 8"
 

themes/sky/theme.lua

@@ -5,7 +5,7 @@
 -- If you want SVGs and extras, get them from garoth.com/awesome/sky-theme
 
 -- BASICS
-theme = {}
+local theme = {}
 theme.font          = "sans 8"
 
 theme.bg_focus      = "#e2eeea"

themes/zenburn/theme.lua

@@ -7,7 +7,7 @@
 --  * http://awesome.naquadah.org/wiki/Nice_Icons
 
 -- {{{ Main
-theme = {}
+local theme = {}
 theme.wallpaper = "@AWESOME_THEMES_PATH@/zenburn/zenburn-background.png"
 -- }}}