fix: detail page missing background (#5303)

before

![CleanShot 2023-12-14 at 14.43.16@2x.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/T2klNLEk0wxLh4NRDzhk/53900c68-c050-4336-80fb-cd121dcf4d53.png)

after

![CleanShot 2023-12-14 at 14.42.44@2x.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/T2klNLEk0wxLh4NRDzhk/bd820718-8032-4a30-b250-6541084830be.png)

packages/frontend/core/src/pages/workspace/detail-page/detail-page.css.ts

@@ -15,6 +15,7 @@ export const mainContainer = style({
   flexDirection: 'column',
   minWidth: 0,
   overflow: 'hidden',
+  background: 'var(--affine-background-primary-color)',
   selectors: {
     [`${root}[data-client-border=true] &`]: {
       borderRadius: '4px',
@@ -47,6 +48,7 @@ export const sidebarContainer = style({
 
 export const sidebarContainerInner = style({
   display: 'flex',
+  background: 'var(--affine-background-primary-color)',
   flexDirection: 'column',
   overflow: 'hidden',
   height: '100%',