Custom Colours for Block Backgrounds

Is your feature request related to a problem? Please describe.
Something that comes naturally to want to make your own notes is always being able to have the freedom to design them how you like, especially with colours. Anytype currently only supports colouring blocks/text with pre-determined colours.

Describe the solution you’d like
Adding the option to allow custom colours via HEX code or RGB values would be very useful, especially for adding a bit more personality / theming to notes.

Describe alternatives you’ve considered
Adding a HEX code option would be the minimum requirement for this feature.

Additional context