Dark mode styles are broken for new transcription/translation source input

[blms]

testing notes (QA)

On the QA site, sign in and visit the page to add a new transcription/translation, with the dropdown to choose the source. After a hard refresh,

• [x] the text in the dropdown should be readable in both light and dark mode
• [x] the button below should appear centered

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dev notes

• Working for me locally but not on QA or prod, suspect some django dependency got an update that broke this. needs more investigation

[kseniaryzhova]

@blms works really well, thank you! Closing!