There are three things in play here.
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The Document, which has its own resolution (size set in the Document menu) and background color (by default a grey to black gradient)
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The Viewport size, determined by UI configuration
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The Viewport background (By default a dark grey–looks like you have changed it to green)
IF the document size is smaller than the viewport frame, zoomed out, or scrolled off center so that only part of the document is visible, you will see the background color of the viewport. This is always an opaque color.
IF the document is LARGER than the viewport, or extends beyond its border in a direction, and IF there is an active tool in EDIT mode, you will see a thin white line between the viewport edge and the main viewing area, that marks a small buffer or “safe zone” in which you can perform navigational gestures there safely that you would normally need empty canvas space to trigger (pan, zoom, etc). This is to ensure people working on large documents while zoomed in on a 3d object always have access to these functions. This is probably what you are referring to.
The entire interface can be made to be transparent against the desktop background with the “See Through” slider on the top right edge of the default UI, but this is probably not what you were talking about.