I understand that you asked about this a while ago, but today I play with the same problem, and I managed to fix it.
Wrap the content inside the div #category-intro-text another div that is relative. Ultimately, you will want to add style to your css, rather than inline, as I did here.
<div id="category-intro-text"> <div style="position: relative;"> <h1>Power Line Markers</h1> Etc. Etc. </div> </div>
Then remove the background color and blending information that you got in the stylesheet for div #category-intro-text . You must end up ...
#category-intro-text { padding: 0.625em 0.938em; position: relative; }
Finally, use the ::before pseudo-element to add a mixed layer.
#category-intro-text::before { content: ""; display: block; height: 100%; position: absolute; top: 0; left: 0; width: 100%; background-color: rgba(220, 235, 255,0.8); mix-blend-mode: lighten; }
Hope it does. It works great for me with a multilayer.
EDIT: Here is a script postponed from the previous answer.
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