Extending themes using Liferay Plugins SDK is simple.
Extending Classic theme is not - there is css/.sass-cache folder to cross your plans.
How to fix? Change build.xml file in your theme folder and override build-css target:
<?xml version="1.0"?>
<!DOCTYPE project>
<project name="test-theme" basedir="." default="deploy">
<import file="../build-common-theme.xml" />
<property name="theme.parent" value="classic" />
<target name="build-css">
<delete file="docroot/css/.sass-cache/main.css" />
<ant
antfile="../build-common-theme.xml"
target="build-css" />
</target>
</project>
