Is this for a single H2? Or all of them?
For single, put <span class="thick1"> around the words, using different class names for each thickness. Then go define the classes in Customizer > Additional CSS or your child theme, to something like
.thick1 {font-weight: 200;}
.thick2 {font-weight: 400;}
.thick3 {font-weight: 700;}
This will usually only render well if the font that you are using has different weights, so you might need to load them in your theme or a plugin.
To affect all H2 elements, you would need to add the classes with a script.
