I've created a new theme. In my _extends.less file I've added the following assignments to exsiting LESS variables;
My understanding is that this should show "Previous" and "Next" labels on the page navigation. It doesn't however make any difference. Like so many other things with Magento I've found a dozen conflicting prices of advice and none of it works. How can I make use of the mixins and variables to style my theme?
You have 2 options:
You need to "compile" the less file in your server (http://lesscss.org/usage/index.html).
Add the script who do it in the browser. Read http://lesscss.org/ and search for "Client-side Usage"
Take a look at this page and see if it helps: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/css-guide/css_quick_guide_approach.html
"Extending a theme using _extend.less is the simplest option when you are happy with everything the parent theme has, but want to add more styles."
I guess that if you want to change the variable you could do a override of that file in your theme.
Copy the original file into your theme and make the variable changes there.