Seva Alekseyev is right, you should store CSS files in assets folder, but referring by file:///android_asset/filename.css URL doesn’t working for me.
There is another solution: put CSS in assets folder, do your manipulation with HTML, but refer to CSS by relative path, and load HTML to WebView by loadDataWithBaseURL() method:
webView.loadDataWithBaseURL("file:///android_asset/", htmlString, "text/html", "utf-8", null);
E.g. you have styles.css file, put it to assets folder, create HTML and load it:
StringBuilder sb = new StringBuilder();
sb.append("<HTML><HEAD><LINK href=\"styles.css\" type=\"text/css\" rel=\"stylesheet\"/></HEAD><body>");
sb.append(tables.toString());
sb.append("</body></HTML>");
webView.loadDataWithBaseURL("file:///android_asset/", sb.toString(), "text/html", "utf-8", null);
P.S. I’ve come to this solution thanks to Peter Knego’s answer.