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How do I make the portfolio titles visible on the portfolio listing templates?

You can use custom CSS to adjust that. Please first make sure you are using the latest version of the theme, then go to "Appearance > Customize > Additional CSS" and you can apply this code to make the titles visible:

.portfolio-listing .portfolio-item h2 {
    left: 0;
    bottom: -30px;
    height: auto;
    clip: auto;
    font-size: 12px;
    text-align: center;
    width:  100%;
}