Improve theme setting

[kjh9348]

Hello, I've made a few changes to the theme setting:

• Recreate activity instead of restarting the app when a user changes theme
• Add "System default" option to follow the system dark mode setting

[Ryosuke839]

Thank you for the great update! I'll include this in the next release.

By the way, I guess we can make "System default" as app default. I'll make a change in another PR.