Fix log font size / color contrast

[dcog989]

Tweaked font colors to conform with WCAG contrast - was less than 2:1 (illegible), now 7:1 for --color-error and --color-danger:.

Also bumped size a little for the log listing, and added 'Source Code Pro' to monospace font stack.

Addresses https://github.com/kopia/kopia/issues/4459