Style AppMap webviews to match the IDE's theme

[jansorg]

Closes #767
Closes #764

This PR provides CSS variables with colors and font properties to the AppMap webviews.
It also applies the IDE's scrollbar CSS style to the AppMap webviews, webview scrollbars should be more similar to the regular scrollbars of the IDE now.

Questions / open issues:

• Is there a property to provide the monospace editor font, e.g. to apply it to code snippets?
• I'm not sure how to style the white AppMap icon in the right panel ("Sequence diagrams")
• This needs testing on a Mac with a HiDPI screen to make sure the IDE's scale factor is properly applied

I decided to not update the styling of an opened webview if the user changes the theme. This won't happen very often and I don't think we should support this at this time.

[dustinbyrne]

It looks like the AppMap icon in the right doesn't adhere to the theme. We can fix this in a future release of the frontend, unless you want to shim it in via something like:

.context__body__table-row__header svg path {
  fill: var(--appmap-color-foreground); 
}

It looks like the monospace font family is simply set to monospace - I'm not sure if JetBrains resolves that in some way or it's solely resolved by the OS.