Fix inability to change accent colour consistently in custom theming (#12772)

Signed-off-by: Michael Telatynski <[email protected]>

Author: t3chguy

res/themes/light-custom/css/_custom.pcss

@@ -18,7 +18,7 @@ $font-family: var(--font-family, $font-family);
 $monospace-font-family: var(--font-family-monospace, $monospace-font-family);
 
 /* Colors from Figma Compound https://www.figma.com/file/X4XTH9iS2KGJ2wFKDqkyed/Compound?node-id=559%3A741 */
-$accent: var(--accent, $accent);
+$accent: var(--accent-color, $accent);
 $alert: var(--alert, $alert);
 $links: var(--links, $links);
 $primary-content: var(--primary-content, $primary-content);