Switch svg colour code to use themes properly

[pokey]

Looks like this should now be possible

[pokey]

Looks like this is actually not possible because of the following:

• The css variables mentioned in Allow SVGs in getting started to be themeable microsoft/vscode#126617 are not actually accessible in main editor, just in a webview
• Even if they were accessible, svg's specified by a uri in the content of a css :after can't access css variables

Probably the best option here if we want to use proper theme colors is the hack described here and here. Could be merged in with our font width measurement hack, with the downside being that we'd need to trigger the webview every time they switch theme (eg dark <=> light), rather than only when they change font family

[pokey]

after discussion with @AndreasArvidsson, deciding it's not worth the hack for now; will reopen if / when proper svg theming becomes possible (microsoft/vscode#32813)