The first Mac OS X version I can remember using was 10.4 (Tiger), and I still love the way it looks. The design changed quite a bit with 10.5 (Leopard), but not necessarily for the worse. The Leopard look is also beautiful in its own right, and I can’t really decide which one I like more.
Starting with 10.7 (Lion) though, Apple started shifting towards a flatter, less colourful look. The previously detailed and colourful sidebar icons turned grey and monochrome, as did the scrollbars. The colourful toolbar icons, like those in app preference windows, held out all the way until macOS 10.15 (Catalina), when almost everything else had already been flattened. They finally disappeared in macOS 11 (Big Sur), when Apple managed to make macOS look even more lifeless, replacing greys with plain whites and swapping out most interface icons for minimal, monochrome SF Symbols.
Missing the old look, I built two tweaks for macOS 12 that let me control the sidebar and toolbar icons.
Sidebars
This tweak scans the app’s user interface and detects sidebar elements, like the cells that list folders och mailboxes. Once found, it swaps their default icons with custom ones defined in a mapping .plist file that links icon identifiers (like labels, file paths, or SF Symbols) to replacement images. Here is how it looks.
Before:
After:
Toolbars
The toolbar tweak works essentially the same way. It scans the toolbar inside the preferences window of apps and swaps their icons according to what is defined in the mapping .plist file. Here is how it looks.
Before:
After:
The “after” screenshots also show a few extra touches from another tweak not detailed here, but essentially it loads the SystemAppearance.car (lifted from macOS 10.14) in order to restore its textures, changes the title bar font to regular, and in Finder it extends the toolbar and status bar.