Dark theme background fix.

Author: Leshe4ka

frontend/src/components/PageContainer/PageContainer.styled.ts

@@ -38,7 +38,7 @@ export const Sidebar = styled.div<{ $visible: boolean }>(
     overflow-y: auto;
     transition: width 0.25s, opacity 0.25s, transform 0.25s,
       -webkit-transform 0.25s;
-    background: ${theme.layout.mainBackgroundColor};
+    background: ${theme.default.backgroundColor};
     transform: ${$visible ? 'translateX(0)' : 'translateX(-100%)'};
 
     @media screen and (max-width: 1024px) {

frontend/src/theme/theme.ts

@@ -352,7 +352,7 @@ export const theme = {
     color: {
       normal: Colors.neutral[90],
     },
-    backgroundColor: Colors.neutral[0],
+    backgroundColor: Colors.brand[0],
     transparentColor: 'transparent',
   },
   link: {
@@ -836,7 +836,7 @@ export const darkTheme: ThemeType = {
     color: {
       normal: Colors.neutral[0],
     },
-    backgroundColor: Colors.neutral[90],
+    backgroundColor: Colors.brand[90],
     transparentColor: 'transparent',
   },
   link: {