I am trying to create a banner looking header for my page and I have trouble changing the font size within the table. When I change, all the fonts in the other tables in other webparts change also.

This is my code:

<style>
table.mmm, th, td {
color: white;
width: 100%;
text-align: left;
font-size: 12px;
}

</style>
<table class="mmm" bgcolor="#5582AB">
   <tbody> 
      <tr>
         <th>Month​</th>
      </tr>
   </tbody> 
</table> 

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What should I have done correctly? Also, is there a similar thing as <strong> </strong> in CSS? 

Thank you!!