Well I managed to solve the problem with overriding standard css rules by inheriting my project's .css file in .gwt.xml file of my project. When you set your user defined .css this way - AFTER the usual inherit line - it will have the higher priority in cascading one rule than the same rule, defined at standard gwt stylesheets.
It took a couple of hours to figure out how to inherit it properly, cause in first try just simply typing <stylesheet src='WebTerminal.css' /> in my .gwt.xml file and commenting out <link type="text/css" rel="stylesheet" href="WebTerminal.css"> in my .html host page didn't bring me any result.
So, the solution is to use relative path, when you set your .css in .gwt.xml config, like this:
<stylesheet src='../WebTerminal.css' />
Notice ../ in the beginning of my relative path. To figure out how it works, add Window.alert(GWT.getModuleBaseURL()); as the first line of your onModuleLoad() method. It will show you something like https://localhost:8080/myproject/resouces/webtermial/, when in fact your hosted page URL would look like https://localhost:8080/myproject/resouces/WebTerminal.html.
Here myproject/resouces is a directory, that contains your .css file, and when you set it in .gwt.xml like <stylesheet src='WebTerminal.css' />, the compiler starts looking for myproject/resouces/webtermial/WebTerminal.css and can't find it. That's why adding ../ sometimes is the only thing to do to solve your problem.
In addition to the words said above I only want to mention that I was not successful in attempt to find any description of this matter in the latest documentary or throughout the discussions taking place at google groups. Wish it was less harder to figure out, because GWT has much more really complex problems itself, than one, which must have had an exhausted description inside tutorial.
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