To access the tool, go to the Look and Feel admin page (tiki-admin.php?page=look), and the "Customization" tab. The activation checkbox is near the bottom of the page. If you don't see it, the reason is probably that Advanced features aren't being displayed, so follow the instructions in the box below.

 Activating Advanced Features

This is an "advanced" feature, so be sure the feature preferences filter is set to "Advanced". You can make this your default in the dropdown list (under the filter icon).

Switch preference filter to Advanced

Check desired preference levels as your personal default, and make sure the "Experimental" field is checked.

Activate the feature by checking the checkbox and clicking the "Apply" button. When the page refreshes, hover over the Theme Customizer "Actions" icon and, in the popup, click " View". This opens the Theme Customizer page.

Theme Customizer Checkbox

The tool works with the theme that is currently active at the site. Shown here are the Slate and TheNews themes.

Slate theme

TheNews theme

There are sections of the tool for these page elements: page colors, fonts, headings, tables, buttons, forms, lists, navbar, dropdowns, tabs, alerts, and icons. At the page bottom there is a "Select a section" link for navigating to the sections in the page.

Section selection

Colors can be selected by clicking in the square sample area for each item, which causes a colorpicker to display. Or the CSS hex value can be edited directly.

Colorpicker

Changes such as font size and style affect the those properties on the HTML body tag, meaning that the changes will be implemented throughout the site unless they are overridden by a more-specfic CSS rule for a particular page element.

After making the desired edits, to save the results as CSS code, click the "Generate Custom CSS" button.

Custom CSS

To use the new custom CSS, copy it from the text area and paste it into the Custom CSS text area on the Look and Feel admin page. Generating custom CSS with this tool actually produces the CSS for all the items on the Theme Customizer page, not just for the edited/changed items. To reduce the amount of redundant CSS rules, the custom CSS could be edited to retain just the changed rules, and the other, unchanged rules could be removed (if you are familiar enough with editing CSS). [This redundancy should be improved in future development.]

As Look and Feel custom CSS, these style rules will of course affect any theme that is in effect at the site, and could have unwanted effects if a theme is selected for use other than the theme whose rules were customized with the Style Guide tool. This is something to consider if more than one theme is used at the site, such as if users have permission to select a theme for their personal use.
(More information on using custom Less variables is coming soon.)