Style updates

[leolost2605]

• Use system accent color instead of a custom one. This was discussed previously in #668. I personally think there is no reason to use a custom accent color (lime green) here but I suppose that's personal preference.
• Slide months right/left instead up/down (Closes #779). Also up to personal preference but I think it makes sense to stay consistent with the arrows in the headerbar and with how the wingpanel indicator handles it.

These changes are a suggestion please reject if they don't make sense designwise :)

[zeebok]

@danirabbit can we consider you adding this to the release as a thumbs up to using user accent color?