Add dark mode #117

Issue: Right now, blog theme only supports light mode. No support for dark mode.
Proposal:

• Add support for Dark mode
• There should be configurable option for user to choose their preference. It can be light, dark and third option is system default.
• The light mode: No changes. Let system use current method without any changes.
• The dark mode: Should be the mode in which blog performs dark theme. In dark theme, user can read in night.
• System default: If user's system has dark mode then it automatically switches to dark or light mode.
• Once user has given their preference, then system should remember their preference and after that whenever they are vising again, they should not have to re-prefer their choice. May be see possibility to store user preference in browser cache or any similar storage and read next time on user visit.
• User should be able to change their preference and change of preference is saved and remembered until next time.