Add Sinistra theme

[roushinkai]

Contributing a theme for a left aligned taskbar, uses 40px taskbar height and 54px button width alongside a show desktop button indicator. It's an alternative if the Windows 11 taskbar takes too much space on your monitor.

[m417z]

Thanks for the contribution. It looks like the most visible effect of this theme is the active item background style (no round corners, full width), and the spacing which can also be customized with the "Taskbar height and icon size" mod.

I'm not sure this small change warrants a dedicated theme. Perhaps it'd be better to have the relevant styles added to the styling guide.

[SandTechStuff]

@m417z I could add this to the styling guide, if you would like.

[m417z]

@SandTechStuff sure, why not, go ahead.