Support / fix dark mode

[pscanf]

Doubts:

• should slide miniatures also be rendered in dark mode?
• should the exported pdf also be rendered in dark mode? (Excalidraw offers an option to export either in light or dark mode, should Excalideck also offer that option?)

[pscanf]

Note: the possibility to toggle Dark Mode / change background colour has been removed in #36 (hiding the control with CSS), figuring it's better to have it off than on and broken. It will be re-added when this issue is solved.

[jcezarms]

Discussing the doubts posed:

1. It makes contextual sense for all widgets to adhere to dark mode once the canvas has switched. This is standard behavior for dark-mode-enabled interfaces, and is going to be expected by most if not all users.
2. Similarly as in (1), the users who eventually use both the dark mode and PDF export features will expect a consistent layout between both. I for one have encountered a similar problem in VSCode's Markdown rendering extension in the past.