Fix a11y violations in nightlies and improve a11y config (#7499)

# Description of Changes
This fixes the a11y violations that are currently failing in the
nightlies in dark mode. Now that we're down to 0 baseline, we can
require the a11y tests to pass in PRs before they merge, so I've changed
that, and I've also made it so that the nightly will report failures in
both light and dark mode instead of just light mode if that fails.

<img width="2560" height="838" alt="image"
src="https://github.com/user-attachments/assets/b0322182-f6ff-4dea-9a1c-5a6c9c9c5439"
/>

frontend/editor/src/core/components/filesPage/VersionTimeline.tsx

@@ -219,7 +219,20 @@ export function VersionTimeline({
                     align="center"
                     style={{ flex: 1, minWidth: 0, flexWrap: "nowrap" }}
                   >
-                    <Badge size="xs" variant={isActive ? "filled" : "light"}>
+                    <Badge
+                      size="xs"
+                      variant={isActive ? "filled" : "light"}
+                      styles={
+                        isActive
+                          ? {
+                              root: {
+                                backgroundColor: "var(--c-accent-solid)",
+                              },
+                              label: { color: "var(--c-text-on-primary)" },
+                            }
+                          : undefined
+                      }
+                    >
                       v{v.versionNumber ?? 1}
                     </Badge>
                     <Text