How DC offset affects the spectrum of a single pulse
Applying an offset to a signal will only change a spectrum at DC (frequency of 0Hz).

If the single square pulse goes from 0V to 8V, the value of DC would be \$\mathcal{F}\{ f(t)\}|_{\omega=0}=0\$. If the square wave goes from -2V to 6V, then \$\mathcal{F}\{f(t)\}|_{\omega=0}=-2\cdot\delta(0)\$.