Fix bug on Avatar.ts for custom theme avatar background color (#10598)

Author: jdauphant

src/Avatar.ts

@@ -96,7 +96,7 @@ export function defaultAvatarUrlForString(s: string): string {
     const colorIndex = total % defaultColors.length;
     // overwritten color value in custom themes
     const cssVariable = `--avatar-background-colors_${colorIndex}`;
-    const cssValue = document.body.style.getPropertyValue(cssVariable);
+    const cssValue = getComputedStyle(document.body).getPropertyValue(cssVariable);
     const color = cssValue || defaultColors[colorIndex];
     let dataUrl = colorToDataURLCache.get(color);
     if (!dataUrl) {