To disable the creation of .ds_store files, launch the Terminal application from /Applications/Utilities/ and enter the following command string exactly:

defaults write com.apple.desktopservices DSDontWriteNetworkStores true

First - open Finder -> Go to folder -> /Library/Caches

The second - find file com.apple.desktop.admin.png and replays it with your image ;) filename need be same = com.apple.desktop.admin.png. You also may rename jpg file to png without converting.

Open Terminal and enter the following in a single line:

/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend

Launch the Terminal, you'll see a list of all your messages. From the mail prompt, do

$ mail
? delete *
? q

And that should be it. Make sure do to the q after the delete * command, that saves the changes to disk.

Here’s the command. Just paste this line into your Terminal, and press return.

defaults write com.apple.screencapture disable-shadow -bool true
killall SystemUIServer

To test out your new screenshot settings, just take a quick screenshot by pressing the key combination command-shift-4, followed by the spacebar. You should end up with a beautifully clean window screenshot, free of drop shadows. The best part about this is that your screenshot’s dimensions will now finally match the window’s dimensions, without all that added padding introduced by the drop shadow. Nice, huh?

If you no longer want these cleaned up windows, you can switch back to the normal shadowed screenshots by pasting the following into your Terminal:

defaults write com.apple.screencapture disable-shadow -bool false
killall SystemUIServer

Well done!

On Windows, I could use Tab to shift focus from button to button, so I could "click" on the desired button without having to use a mouse.

It appears that I can't do the same thing on a Mac, or is there a way to do it that I didn't realize?

There is. From System Preferences, under Keyboard » Keyboard Shortcuts, you can enable Full Keyboard navigation. You can also change this by pressing Control+F7

Hi there! Run command to your Firefox (since v44~)

about:config

about:config => dom.push.enabled
dom.webnotifications.enabled => false

// and all messages, where you can see Push :)