[MM-54750] Theme color sidebarHeaderTextColor is used with different backgrounds (#26085)

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/components/sidebar/sidebar_header/sidebar_header.tsx

@@ -62,7 +62,7 @@ const SidebarHeading = styled(Heading).attrs(() => ({
     margin: 'none',
     size: 200,
 }))<SidebarHeaderProps>`
-    color: var(--sidebar-header-text-color);
+    color: var(--sidebar-text);
     cursor: pointer;
     display: flex;