In System Settings go to Accessibility - then Display - then activate Reduce transparency

A macOS Sonoma 14.7 theme that has some Aqua elements in it

PREVIEW - subject to change

text edited and snagged from jslegendre https://github.com/jslegendre/ThemeEngine

Note for Big Sur, Monterey and Ventura users: If you want to edit system .car files it is no longer enough to remount the boot volume as read/write. You will need to follow the steps below:

1. Disable SIP and authenticated-root: In recovery mode, open Terminal and enter

csrutil disable

csrutil authenticated-root disable

Then reboot into macOS.

2. Identify the System Volume Disk Device Name Run the mount command in Terminal to identify the devices of your system volume snapshot. Look for the device mounted at “/“, which identifies the system volume disk. In the following example, this disk is /dev/disk4s5s1.

/dev/disk4s5s1 on / (apfs, sealed, local, read-only, journaled)
devfs on /dev (devfs, local, nobrowse)
/dev/disk4s4 on /System/Volumes/VM (apfs, local, noexec, journaled, noatime, nobrowse)
/dev/disk4s2 on /System/Volumes/Preboot (apfs, local, journaled, nobrowse)
/dev/disk4s1 on /System/Volumes/Data (apfs, local, journaled, nobrowse)

To get the actual name of the system volume disk, remove the final “sX” from the device. In the preceding example, the name of the system volume disk is /dev/disk4s5.

3. Mount a Live Version of the System Volume Run the mount command in Terminal to mount the system volume disk to a temporary location. When running the mount command, always include the nobrowse mount option to prevent Spotlight from indexing the volume.

mkdir /Users/<YOUR USER NAME>/livemount

sudo mount -o nobrowse -t apfs /dev/disk4s5 /Users/<YOUR USER NAME>/livemount

4. cd ~/Downloads/AquaLickX-main/
5. sudo cp Aqua.car ~/livemount/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/
6. sudo cp VibrantLight.car ~/livemount/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/
7. sudo cp DarkAqua.car ~/livemount/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/
8. sudo cp VibrantDark.car ~/livemount/System/Library/CoreServices/SystemAppearance.bundle/Contents/Resources/
9. Bless your livemount directory with sudo bless --mount /Users/<YOUR USER NAME>/livemount --bootefi --create-snapshot

If you're on an Apple Silicon Mac then issue this bless command instead

sudo bless --mount "$HOME/livemount/System/Library/CoreServices/" --setBoot --create-snapshot

Reboot your Mac using the regular reboot mechanisms immediately after issuing the bless command described above.

the command below will reboot your Mac then and there, so be careful unless you're certain that you have all your work saved in other applications

type sudo reboot in the existing Terminal window