Change ghost post width

Learn from the internet, we can simply apply math in CSS.

<style type="text/css">
    .gh-canvas {
        display: grid;
        grid-template-columns: [full-start] minmax(max(4vmin,20px),auto) [wide-start] minmax(auto,240px) [main-start] min(1200px,calc(100% - max(8vmin, 40px))) [main-end] minmax(auto,240px) [wide-end] minmax(max(4vmin,20px),auto) [full-end];
    }
</style>

And the magic is done!