Using other units than pixels for $v-font-size

Hello,

i am using Vaadin-Version 7.5.6 and the Valo theme.
I tried to use "mm" as unit for the variable $v-font-size (Example: $v-font-size: 15mm;) but then i got many IncompatibleUnitsExceptions.
I tried other units like "cm" or "in" but it seems that only "px" are working without exception.
In the Book of Vaadin is written, that "It should be specified in pixels.".
"Should" - Not "have to" (must).
So it should be working with other units too.
Is it a bug?

Here is one of the errors:

INFO: Installed AtmosphereInterceptor  Track Message Size Interceptor using | with priority BEFORE_DEFAULT
Sep 29, 2015 5:06:45 PM com.vaadin.sass.internal.tree.controldirective.IfElseDefNode traverse
SEVERE: null
com.vaadin.sass.internal.expression.exception.IncompatibleUnitsException: Incompatible units found in: '-35mm'
    at com.vaadin.sass.internal.parser.LexicalUnitImpl.checkAndGetUnit(LexicalUnitImpl.java:409)
    at com.vaadin.sass.internal.parser.LexicalUnitImpl.minus(LexicalUnitImpl.java:392)
    at com.vaadin.sass.internal.expression.BinaryOperator$10.evalInternal(BinaryOperator.java:144)
    at com.vaadin.sass.internal.expression.BinaryOperator.eval(BinaryOperator.java:228)
    at com.vaadin.sass.internal.expression.BinaryExpression.eval(BinaryExpression.java:48)
    at com.vaadin.sass.internal.expression.ArithmeticExpressionEvaluator.evaluate(ArithmeticExpressionEvaluator.java:116)
    at com.vaadin.sass.internal.parser.SassExpression.evaluateFunctionsAndExpressions(SassExpression.java:220)
    at com.vaadin.sass.internal.tree.RuleNode.traverse(RuleNode.java:108)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:214)
    at com.vaadin.sass.internal.tree.KeyframeSelectorNode.traverse(KeyframeSelectorNode.java:49)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:214)
    at com.vaadin.sass.internal.tree.KeyframesNode.traverse(KeyframesNode.java:54)
    at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverse(TemporaryNode.java:71)
    at com.vaadin.sass.internal.visitor.IfElseNodeHandler.traverseChild(IfElseNodeHandler.java:66)
    at com.vaadin.sass.internal.visitor.IfElseNodeHandler.traverse(IfElseNodeHandler.java:45)
    at com.vaadin.sass.internal.tree.controldirective.IfElseDefNode.traverse(IfElseDefNode.java:49)
    at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverse(TemporaryNode.java:71)
    at com.vaadin.sass.internal.visitor.MixinNodeHandler.replaceMixinNode(MixinNodeHandler.java:75)
    at com.vaadin.sass.internal.visitor.MixinNodeHandler.replaceMixins(MixinNodeHandler.java:45)
    at com.vaadin.sass.internal.visitor.MixinNodeHandler.traverse(MixinNodeHandler.java:34)
    at com.vaadin.sass.internal.tree.MixinNode.traverse(MixinNode.java:116)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverseChildren(TemporaryNode.java:53)
    at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:92)
    at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverseChildren(TemporaryNode.java:53)
    at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:92)
    at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverseChildren(TemporaryNode.java:53)
    at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:92)
    at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.controldirective.TemporaryNode.traverseChildren(TemporaryNode.java:53)
    at com.vaadin.sass.internal.visitor.ImportNodeHandler.traverse(ImportNodeHandler.java:92)
    at com.vaadin.sass.internal.tree.ImportNode.traverse(ImportNode.java:99)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:227)
    at com.vaadin.sass.internal.tree.Node.traverseChildren(Node.java:214)
    at com.vaadin.sass.internal.ScssStylesheet.traverse(ScssStylesheet.java:294)
    at com.vaadin.sass.internal.ScssStylesheet.compile(ScssStylesheet.java:263)
    at com.vaadin.sass.internal.ScssStylesheet.compile(ScssStylesheet.java:238)
    at com.vaadin.server.VaadinServlet.compileScssOnTheFly(VaadinServlet.java:1096)
    at com.vaadin.server.VaadinServlet.serveOnTheFlyCompiledScss(VaadinServlet.java:1028)
    at com.vaadin.server.VaadinServlet.serveStaticResourcesInVAADIN(VaadinServlet.java:738)
    at com.vaadin.server.VaadinServlet.serveStaticResources(VaadinServlet.java:702)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:344)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:684)
    at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:503)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
    at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:557)
    at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
    at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
    at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:429)
    at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
    at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
    at org.eclipse.jetty.server.Server.handle(Server.java:370)
    at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:494)
    at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:971)
    at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1033)
    at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644)
    at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
    at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:696)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:53)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
    at java.lang.Thread.run(Thread.java:722)

Thanks for your help.