Injecting dynamic variables to rewrite relative URLs

We have a somewhat esoteric case where we use styled-jsx inside a Cordova app (via a `WKWebView`. Cordova apps load their index page from a local file, so `window.location` points to some file URL like `file:///Applications/...`.

While CSS loaded via `link` is fine, the `file://` URL has the side effect that relative URLs used in styled-jsx code won't work:

```jsx
{`
.thing {
background: url(/thing.png);
}
`}
```

We somehow have to change this to be an absolute URL:

```jsx
{`
.thing {
background: url(${Config.assetBase}/thing.png);
}
`}
```

...where `Config.assetBase` is a per-environment configuration. We can't define this string as a SASS variable globally because it's dependent on the current page. (We create a single Docker image for all environments.)

The above would be fine _if_ all our code used styled-jsx, but we also have a ton of shared `.scss` files we import that contain mixins and stuff that need to be augmented this way. We _could_ define `base` for all the imports:

```jsx
{`
$base: ${Config.assetBase}`;
@import "stuff"; // This now has access to $base
.thing {
background: url(${base}/thing.png);
}
`}
```

~~This works~~. This doesn't work because styled-jsx doesn't support string interpolation. Unfortunately, this means _every_ styled-jsx component needs to define this variable.

Is there a way to inject SASS variables _dynamically_ and _not_ at transpilation time? We can't do it during Babel transpilation because it's already too late — the base URL is effectively per-request, not a compile-time setting. Or is there a way to post-process the generated CSS so we can change relative URLs to be absolute?

Any ideas?

The issue names no source files, tests, or entry points; begin by locating the plugin's Sass processing and how styled-jsx handles runtime interpolation. Before implementation, establish whether dynamic Sass variables or post-processing is supported and define a testable acceptance criterion for rewriting relative URLs.

Written by the indexing model from the issue text.