I would suggest you to write your own highlighting.
In this case you should edit file filetype_extensions.conf
, create a file filetype.less
(see other filetypes for examples, preferably those similar to .less - like .css) and maybe play around with filetype.conf
.
On the other hand, if you never work with .css files, you could just edit corresponding configuration files to set up the highlighting you would enjoy.
If this is still not enough, you may have take another editor (with designed highlighting for .less files), see how the highlighting is organised there and then reproduce it with geany.
This may be overcomplicated solution, though I don't insist it's the only one.