This is the simple CSS to highlight DIV.

.highlight { background: darkblue; }

Now check out the 3 DIVs below.

• For 1st Div I extend .highlight with .highlight_desktop so .highlight class apply to this DIV in desktop mode (if screen is higher than 1024).
• For 1st Div I extend .highlight with .highlight_tablet so .highlight class apply to this DIV in tablet mode (if screen is higher than 640 and lower then 1024).
• For 1st Div I extend .highlight with .highlight_mobile so .highlight class apply to this DIV in mobile mode (if screen greter than 0 and lower then 640)

Resize your browser then refresh and check the result