Batch size finder code example in dark mode is light instead of dark

Nobody has claimed this yet.

### 📚 Documentation

On the [Batch size finder advanced tricks](https://lightning.ai/docs/pytorch/stable/advanced/training_tricks.html#batch-size-finder) page, in dark mode the example code is in light mode makes it hard to read:

The code example should look instead look like this in dark mode:

cc @lantiga @borda

Open the Batch size finder advanced tricks page at the linked training_tricks.html#batch-size-finder section and inspect how its code example and dark-mode styling are authored. Reproduce the page in dark mode, update the documented example or its styling so the code is readable, and verify that the example renders correctly in both light and dark modes.

Written by the indexing model from the issue text.