* lisp/textmodes/css-mode.el (css-nmchar-re): Allow underscores in
variable names (and in identifiers in general).
* test/manual/indent/css-mode.css: Add some examples of variable names
with underscores in them.
* test/manual/indent/less-css-mode.less: Add some examples of variable
names with underscores in them.
* test/manual/indent/scss-mode.scss: Add some examples of variable
names with underscores in them.