Answer
$$\theta=\frac{\pi}{2}.$$
Work Step by Step
Since $ n_1=\langle 1,0,1\rangle $ and $ n_2=\langle -1,1,1\rangle $, we have
\begin{align*}
\cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\
&=\frac{-1+0+1}{\sqrt{1+0+1}\sqrt{1+1+1}}\\
&=\frac{0}{\sqrt{2}\sqrt{3}}\\
&=0.
\end{align*}
That is $$\theta=\frac{\pi}{2}.$$
