Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 5.4.0
• Cross-platform modules: 5.4.0
• Android Runtime: 5.4.0
• iOS Runtime: 5.4.0
• Plugin(s):

Describe the bug
The NativeScript theme provides some nice helper classes for margin and padding. I wanted to apply a margin-top of 5 to my button. I've used m-t-5 which simply did not work on my button. When I've checked, we've already specified margin: 0 for our button (.btn) class. So first I thought it might be a problem of the the order the classes are applied but I could move them around without any success. For some reason I came up with a solution by adding m-0 to the button as well which does not make any sense for me but it works.

To Reproduce

CSS:

.btn {
    margin: 0;
}

Layout:

<StackLayout class="home-panel">
    <Button class="btn btn-primary" text="Button"></Button>
    
     <!-- Expecting here a margin of 5 at the top, but does not work -->
     <Button class="btn btn-primary m-t-5" text="m-t-5"></Button>

      <!-- This is working for some reason, by adding 'm-0' as additional class, but
	     that doesn't make sense since '.btn' has also 'margin: 0' defined -->
      <Button class="btn btn-primary m-t-5 m-0" text="m-t-5 & m-0"></Button>
 </StackLayout>

Expected behavior
My expectation: m-t-5 (or m-t-N) should overwrite the margin: 0 of the .btn class we specified. The current solution is actually something I'd say is incorrect - since m-0 should overwrite m-t-5

Sample project

I've created a pretty simple playground to demonstrate the problem:
https://play.nativescript.org/?template=play-ng&id=ad5EUa&v=3

Only needed to add the css as described above and changed the layout in the home component.

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