fix(core): border color for embed-linked-doc-block and others (#13056)

fixes https://github.com/toeverything/AFFiNE/issues/12972

border: `#eee` -> `#f3f3f3`

background: `#fff` -> `#fff` (no-op)

This change is based on [@fundon 's PR from 2 month
ago](https://github.com/toeverything/AFFiNE/pull/11763/files#diff-cc768d5886dd743d8a7ad97df05added2710c0487d281f2b33b02ab1a9c78e4c)
which I assume has the most up-to-date design

I am also curious to know the current state of CSSVarV2 function. Should
it replace all previous usage of css variables? I can do a find-replace
globally (not just embed blocks) if that's important enough.

<!--

https://github.com/toeverything/design/blob/main/packages/theme/src/v2/variables.ts#L296
-->

blocksuite/affine/blocks/image/src/preview-image/edgeless.ts

@@ -1,3 +1,4 @@
+import { unsafeCSSVarV2 } from '@blocksuite/affine-shared/theme';
 import { toGfxBlockComponent } from '@blocksuite/std';
 import { css } from 'lit';
 
@@ -9,7 +10,7 @@ export class ImageEdgelessPlaceholderBlockComponent extends toGfxBlockComponent(
   static override styles = css`
     affine-edgeless-placeholder-preview-image
       .affine-placeholder-preview-container {
-      border: 1px solid var(--affine-background-tertiary-color);
+      border: 1px solid ${unsafeCSSVarV2('layer/background/tertiary')};
     }
   `;