Add colour variables to SVGs

[samrichca]

SVGs on Overture.bio (i.e. in the Icon component) currently have hard-coded colour hex codes. Using CSS variables to define the colours (with a fallback for Edge) would ensure better visual consistency and reduce the level of effort for future maintenance.

[samrichca]

we could change the SVG files to JSX so they take props & colour schemes, and use styled components or CSS-in-JS, similar to ego, argo, etc:

https://github.com/overture-stack/ego-ui/pull/118/files

changing the components and where styling is located would impact this ticket:

#112