Formatting helios.less
Making themes customizable

Legacy IDE as well as Helios look for
`Smalltalk vm at: 'codeMirrorTheme'` before setting their defaults.

Helios defaults are 'default helios', legacy IDE defaults are 'default'.
Extracting Helios fonts and colours.

Helios's colour and fonts settings are in CodeMirror helios theme.
Hint selection menu should have sans-serif menu font.

IMO. I'm not going to fight for this.
#helios -> body[id="helios"]
Change to use `SmalltalkSettings`.
SmalltalkSettings at: 'helios.completionKey'
Merge pull request #829 from herby/helios-css

Helios and legacy IDE theming customizable