Calendar Pro
Six more looks for Calendar on top of dark and light: Midnight, Graphite, Forest and Plum for dark rooms, Paper and Ocean for bright ones. Your accent colour and event colours work with every theme. Part of Calendar Pro.

Click the cog at the top right, or Themes pack under Calendar Pro in the sidebar.
Choose one from Calendar Pro themes in the Theme list. It changes straight away.
Pick Light or Dark, or choose Dark mode / Light mode in the ··· menu.
Appearance help has every option in detail.
Part of Calendar Pro. Calendar is free. Calendar Pro is a one-off £5.99 upgrade inside the Windows app, bought through the Microsoft Store - no subscription, no account. Try it free for 7 days.
Dark and light themes, six accent colours and solid or soft event colours are free. The six extra themes are Calendar Pro.
Yes - Calendar Pro's 7-day free trial includes them.

Day, Week, Timeline, Month and Agenda - drag to reschedule, week numbers and UK bank holidays.

Tag events by project, filter to one, and log actual hours against planned.

Notes for every day, exported as a formatted site diary for any date range.

Windows notifications before your events, even when Calendar is closed.

Day, week, month and year wall planners in three styles - print or PDF.

Every event for a project, planned against logged hours, with CSV export.

Diary entries, weekly update emails and project summaries - private, on your PC.

All twelve months at once, with event dots - click a day to open it.

Bank holidays for Scotland, Northern Ireland and Ireland, and the clock changes.

Weekly and monthly timesheets from your hours, and an invoice from them.

Everything in one file - restore with a preview, merge or replace, and undo.
Calendar is free, in your browser and on Windows. Calendar Pro is a one-off £5.99 in the Windows app, with a free 7-day trial.