How to change post featured image overlay color in Oblique

To ensure that your change is not lost when you update the theme then please create a Child theme:  How to create a child theme and add the following CSS to the your custom.css. You could also install a Custom CSS plugin and add the snippet to it:

.entry-thumb:after {
    background-color: rgba(252, 248, 227, 0.52);

You could generate your own color here: Hex Color Tool