Fix Technology scalability image for dark theme

[matevz]

Followup to #1055

Fix colors of
https://github.com/oasisprotocol/docs/blob/main/docs/general/images/architecture/technology_scalability.svg

Currently looks like this:

This is a very old diagram (exported from Adobe Illustrator or Inkscape?) which received gradual updates over years. I suggest we either redraw it in drawio.com and use its theming or temporarily add a CSS section inside the existing SVG file for the dark theme.