how do certain variable fallbacks get injected

[dsmmcken]

Just curious how this works,. Some variables get a fallback(?), but I don't understand where they come from.

For example:
in adobe/spectrum-css-temp/components/table/skin.css

I can see it's defined as:

  &.is-selected {
    .spectrum-Table-cell {
      background-color: var(--spectrum-table-row-background-color-selected);
    }

No where do I see --spectrum-table-row-background-color-selected defined. And then the output becomes:

background-color: var(--spectrum-table-row-background-color-selected-hover,var(--spectrum-alias-highlight-selected-hover))

Where and how does --spectrum-alias-highlight-selected-hover get added? I see a bunch of post processing steps, but I don't see where that mapping would logically come from. Searching for alias.*selected doesn't reveal anything, so is the mapping from another an external package?

Would appreciate any insight, just trying to get a clear picture of how this all works, thanks.

[reidbarber]

I believe these are coming in at build time from the @spectrum-css/vars package via the postcss plugin at @spectrum-css/component-builder: https://github.com/adobe/react-spectrum/blob/main/postcss.config.js