To apply this theme to all Tk applications, add

*TkTheme: Arc

e.g. to your ~/.Xresources file, then merge it to the X resource database by
calling

$ xrdb -merge ~/.Xresources

You may want to add this line to your ~/.xinitrc or window manager specific
X session startup file if it isn't in there yet.

It's also possible to apply a certain theme only to windows of a specific
class. The default class Tk assigns to all toplevels is the name of the
application. For example, if you only wanted the Arc theme to be
applied to all windows of the "gitk" tool, add

*gitk.TkTheme: Arc

to your ~/.Xresources instead.
