chore(docs): update theming-v1 links (#2248)

Updates a few dead links in the legacy theming guide.

Author: oliverlaz

docusaurus/docs/React/guides/theming/css-and-theming.mdx

@@ -106,7 +106,7 @@ that makes the list UI perform optimally.
 ### CSS Variables
 
 To make more sweeping stylistic changes to the default CSS, the library exposes a variety of
-[CSS variables](https://github.com/GetStream/stream-chat-css/blob/main/src/styles/_variables.scss) that can be
+[CSS variables](https://github.com/GetStream/stream-chat-css/blob/main/src/v1/_variables.scss) that can be
 adjusted with props. The `Chat` component accepts a `customStyles` prop, an object type with keys as the CSS variables
 you wish to change and values for the corresponding, adjusted CSS.
 
@@ -190,6 +190,6 @@ const darkModeTheme: CustomStyles = {
 #### Variables Reference
 
 A complete reference of all supported CSS variables, as well as their default values, can be found within our
-[SCSS variables file](https://github.com/GetStream/stream-chat-css/blob/main/src/styles/_variables.scss).
+[SCSS variables file](https://github.com/GetStream/stream-chat-css/blob/main/src/v1/_variables.scss).
 Any of the CSS variables (prepended with `--`) can be overridden via the `customStyles` approach detailed
 above.