Same background colors for Custom Containers and Code Blocks in dark theme

When viewing the docs in dark theme, both Custom Containers and Code Blocks have the same background color of `var(--vt-c-bg-soft)`. When Code Blocks are nested in Custom Containers (see [example](https://vuejs.org/guide/components/v-model.html#under-the-hood)), the colors can be confusing.

I also checked VitePress -- it seems that the issue is with the custom theme of the vuejs/docs repo.