Allow for custom themes

[shilangyu]

Is your feature request related to a problem? Please describe.

Inspired by this post. More theming options not only allow for a more personal look but also would solve individual issues like described in the linked post.

Describe the solution you'd like

Allow for creation of custom themes. Font size, particular colors, spacings, etc.

• [ ] An interface where many visual aspects of lemmur can be changed
• [ ] A theme can be exported to a portable file
• [ ] A previously exported theme can be imported

Describe alternatives you've considered

Create some preset themes.

Valuable resources

• https://pub.dev/packages/flex_color_scheme