RenderThemeGtk.cpp improperly uses color icons in paintSearchFieldResultsDecorationPart(), paintSearchFieldCancelButton(), and paintCapsLockIndicator(). The proper icons would be edit-find-symbolic, edit-clear-symbolic, and dialog-warning-symbolic, respectively. I tried to fix this by calling getStockSymbolicIconForWidgetType() rather than getStockIconForWidgetType() in those functions, but the icons in the search entry didn't display unless I removed the flag GTK_ICON_LOOKUP_FORCE_SIZE from the call to gtk_icon_theme_lookup_icon() in getStockSymbolicIconForWidgetType(), and then they displayed blurry, which is weird for symbolic icons, so I was doing something wrong. Testcase for the search entry is to open the web inspector and look in the upper right.
Better testcase: the search box on https://www.wikipedia.org/ The magnifying glass icon should be monochrome, not blue.
Fixed in r194847