[Feature Request] Automatic Theme Toggle

Hi @yury & @carloscabanero 👋

Thank you for this awesome app.

Similar to iPadOS' automatic toggle between light and dark mode, I am wondering if it is possible to have Blink toggle between light and dark themes (or toggle between any 2 themes based on time of day).

For my use-case, I generally prefer lighter themes during the day as it is easier to read black text on white background in bright environments and switch to darker themes at night as it's softer on the eyes in dark environments.

macOS Catalina also supports automatic toggling between light and dark mode as well, which extends to Apple's terminal app as well. Tied with #222, this feature would make for a really nice end-to-end light/dark mode experience on macOS as well as iPadOS, from the overall system all the way down to the vim process running in the terminal.

The issue names no files, tests, or entry points. Start by reviewing Blink's existing theme-selection and system-appearance behavior, then determine how time-of-day switching should be configured. Done means the app can switch between two selected themes automatically and the behavior is covered by appropriate validation.

Written by the indexing model from the issue text.