Allow background changes to apply immediately

[polhuang]

Currently, changing seoul256-background or running your background-change helper functions only works for subsequent frames, not the existing frame.

Adding this snippet allows any such changes to immediately apply to the current frame:

(if (member theme custom-enabled-themes)
    (let ((custom--inhibit-theme-enable nil))
      (cherry-seoul256-apply theme style dark-fg light-fg dark-bg light-bg))
  (cherry-seoul256-apply theme style dark-fg light-fg dark-bg light-bg))

Let me know if you have any questions or if I'm mistaken in this.

Thanks for your hard work on this port.