To change the background colour of the edit, save and preview buttons, you have to change the CSS.
.editButtons button {
background-color: yellow;
}
For CSS colours have to be in either, keyword, hex, hsl, hsla, rgb or rgba format. Here's a good colour picker for CSS.
Use the .toolbutton
selector to target the buttons for editing the page like bold, italics and underline, and the button
selector to target every button on the page.
You could put your changes at the top of an existing css file or create a new css file just for your edits and include it in the style.ini
file from your current template.