Dilogarithm integral of the form $\operatorname{Li}_2(\sin^8x)$

Evaluate $\frac{1}{\pi}\int_0^{\frac{\pi}{2}}\operatorname{Li}_2\left(\sin^8 x\right)dx$

The challenging work is to find the closed form for the original integral above .

Notation: $\operatorname{Li}_2(x)$ is dilogarithm function.