None of the previous answers worked for me but, with Mojave, this one did the trick.
Instead of doing Command+Shift+3:
In the Options, make sure "Desktop" or "Other Location" is selected
Also make sure that "Remmember Last Selection" is checked
Perform a "Capture"
Then, next time you will do Command+Shift+3, it should be working.
just use 2 commands:
defaults write com.apple.screencapture name "screenshot_$(date +%Y_%m_%d-%H_%M_%S)"
defaults write com.apple.screencapture "include-date" 0
you can also change the default screenshot location (in my case this is a folder in iCloud)
defaults write com.apple.screencapture location "~/Library/Mobile\ Documents/com\~apple\~CloudDocs/...
Choose the fill colour button on the toolbar (red filled square in your screenshot), then open the colour palette with the button at the bottom of the popover, and set the opacity on the colour using the slider or entering a percentage (100% is fully opaque, 0% is fully transparent).