change content of SVG text node via CSS :before

Question

Within HTML I've a SVG block with text nodes within. I want to change the text-node's content via CSS :before or :after. If done so, there isn't any effect.

Here is the HTML snipped:

<defs>
    <symbol id="mytext"><text class="mytext" x="455" y="290">my added text is: </text></symbol>
</defs>

<use x="10" y="10"  xlink:href="#mytext" /></use>

Now I want to append the text tag with the id 'mytext' with CSS like this:

.mytext:after{
  content: "this text";
}`

I do not want to change any style, my only goal is to append content to the SVG's textnode.