You’re perfectly right, why not just do it yourself.
I used the chrome extension “StyleBot” to adapt the stylesheet a bit. It’s so easy that I’m almost embarrassed having asked here … 
That’s the result after just about 10 minutes:
And if anyone is interested, here’s the StyleBot css export:
{“raspberrypi”:{"_enabled":true,"_rules":{“main.mdl-layout__content.mdl-color–grey-100”:{“background-color”:"#575757",“color”:"#cccccc"},“div.mdl-layout__header-row”:{“background-color”:"#2a43a6"},“h5.mdl-form__title.mdl-cell.mdl-cell–12-col”:{“background”:“linear-gradient(45deg, rgba(59,103,158,1) 0%,rgba(43,136,217,1) 50%,rgba(32,124,202,1) 51%,rgba(125,185,232,1) 100%)”,“border-style”:“solid”,“border-top-width”:“0”,“border-right-width”:“0”,“border-left-width”:“0”,“border-bottom-width”:“2pxpxpxpx”,“border-color”:"#616c99",“width”:“100%”},“div.mdl-form.mdl-color–white.mdl-shadow–2dp.mdl-cell.mdl-grid.mdl-cell–12-col”:{“background-color”:"#404040"}}}}