Add an alternative CSS style for deep-color background (like black)

[agentzh]

Please base on the agentzh git branch. Thanks a lot!

[kindy-aa]

@agentzh can you have a look on the new style. you can try it with the "Color" button in the toolbar. it's a toggle button.

[agentzh]

@kindy-aa That's awesome! Thanks a lot! But can we make the blue, red, and pink colors in dark mode a bit brighter?

[kindy-aa]

update again.

[agentzh]

@kindy-aa blue and red are much better in the dark mode now. Thanks! But will you please make pink a bit further brighter? Thanks!