Problem: when having multiple (dark-themed) terminals side by side I find the content hard to read

Solution: increase padding (by a fixed amount of pixels)

$ cat ~/.config/gtk-3.0/gtk.css:
VteTerminal,
TerminalScreen,
vte-terminal {
    padding: 10px 10px 10px 10px;
    -VteTerminal-inner-border: 10px 10px 10px 10px;
}

Source: askubuntu.com