UI fix: dynamic drawing board canvas/colors for light/dark modes

[AbigailDawson]

Because the drawing board canvas color changes depending on dark vs. light mode, it’s difficult for people to collaborate or view the drawing if they’re using different modes. For example, someone using dark mode will use white/light colors to draw on the dark canvas, but for someone using light mode the colors will be nearly invisible (and vice versa).

I think the quickest solution would be to use the same drawing board canvas color for both dark and light mode users; then, the drawing colors can appear the same for all viewers regardless of their light/dark mode setting.

[huly-github-staging]

Connected to Huly®: UBERF-9267

[Chunosov]

#7806