RevealJS code block animations not working on several themes

Bug description

Some RevealJS themes (beige, league, sky) don't support animation of code line number highlighting per https://quarto.org/docs/presentations/revealjs/#line-highlighting (see the example where one can advance with space-bar to show highlighted lines). In the faulty themes, it seems the "background" code is not properly grayed out (so the highlighted code appears to stand out).

Here's a MWE for the problem:

---
format: 
    revealjs: 
        theme: beige # doesn't work
        # theme: blood # works
        # theme: dark # works
        # theme: default # works
        # theme: league # doesn't work
        # theme: moon # works
        # theme: night # works
        # theme: serif # works
        # theme: simple # works
        # theme: sky # doesn't work
        # theme: solarized # works
---

# Animated slide

```{.python code-line-numbers="|6|9"}
import numpy as np
import matplotlib.pyplot as plt

r = np.arange(0, 2, 0.01)
theta = 2 * np.pi * r
fig, ax = plt.subplots(subplot_kw={'projection': 'polar'})
ax.plot(theta, r)
ax.set_rticks([0.5, 1, 1.5, 2])
ax.grid(True)
plt.show()
```

This could very well be a bug in the highlight.js or even the CSS of the themes. I didn't have time to dig deeper, but wanted to document it.

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
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