Are small spaces filled with black?

No. Nothing a reader would color is ever filled with black. Only tiny specks where lines meet (under 0.6 mm²) are closed, because they print as ink anyway. Spaces a little under the readers' limits (mostly narrow corners) stay white and are counted: "N small spaces (under the readers' limits, mostly narrow corners) stay white: a fine pen colors them. Nothing is filled with black." A design with too many of them is drawn again automatically. If you want a small space filled anyway, use "Fill space" in the page editor.