When I open JCE for editing an article or category, I see the editor icons in B/W and not in colour like I saw on a screen shot in the "editor toolbar" documentation. I must be missing something.

The documentation needs updating as it is still showing the Classic icon set. The B/W icons are high-resolution icons for retina displays, so best suited for modern screens. You can switch to the Classic icon set in Editor Profiles -> Features & Layout.

Also the default font size is 12px, which I seem unable to change.
I am using Helix Ultimate 2.0.8 as template for my site.

The font size is set by your template, usually in the body tag. Ideally any changes to the font size should be done to the template stylesheet (you can probably do this using the Helix configuration tools) rather than changing font size and styles in the editor.

Helix Ultimate oesn't appear to write custom changes to a file, but rather renders them as inline styles on the page, which is why they are not picked up by JCE. You will need to create an editor.css file for JCE to use, in the templates/shaper_helixultimate/css/ folder, and add the following:

@import url('bootstrap.min.css');
@import url('template.css');

/* if you are using a preset, add the preset number here, eg: */
@import url('presets/preset1.css');

/* add your typography changes here, eg: */
body{
    font-family: 'Arial', sans-serif;
    font-size: 14px;
}

JCE will then detect and load this editor.css file instead of the others.

This is working in my tests with your editor.css file.

Please send me a login - https://www.joomlacontenteditor.net/contact/site-login

The Editor Styles option in the Editor Global Configuration was set to Default but it should be Template CSS File. I have set this option on your site and the stylesheets appear to be loading as expected.