Image:

Your browser's rendering:

$x\colorbox[rgb]{0,0,0.6}{$$}M\colorbox[rgb]{0,0,0.6}{$$}x\colorbox[rgb]{0,0,0.6}{$$}$

Source Code:

    <math> 
      <mrow> 
        <mi>x</mi> 
        <mstyle background="#000099"> 
          <mspace height="1ex" width="1ex"/> 
        </mstyle> 
        <mi>M</mi> 
        <mstyle background="#000099"> 
          <mspace height="1ex" depth="1em" width="1ex"/> 
        </mstyle> 
        <mi>x</mi> 
        <mstyle background="#000099"> 
          <mspace depth="2mm" width=".2in"/> 
        </mstyle> 
      </mrow> 
    </math>