fix: don't apply extra image styling to images outside the main

Without this modifier, the border, background color, etc, also applies
to logos in the header and footer, and that's not what we want.

website/src/css/custom.css

@@ -61,7 +61,7 @@ html[data-theme='dark'] {
     --unleash-img-background-color: #fff;
 }
 
-img {
+main img {
     background: var(--unleash-img-background-color);
     display: block;
     margin: auto;