On the topic of how to get to the built in / original / default obsidian.css this worked for me on Obsidian v0.14.2, mac os 12.3 Monterey:
- Turn off custom theme to switch to Default Obsidian theme: Settings → Appearance → Themes: Manage → select Default ⇒ click Use button
Result: Obsidian app should be in default theme
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Open web dev tool inspector anywhere in the app: press CTRL + SHIFT + I or Mac users press option + command + i
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Select the Sources tab in Dev Tools → top → obsidian.md → app.css
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Copy & paste into a text editor or right-click and Save as… onto your machine
