Update HTML UI styling

Close for QQ 17.0.0. 

A set of patches has been submitted to gerrit, to start using a color scheme that matches the flutter sample HMI. Affected applications: homescreen, launcher, hvac, dashboard, media player.

In order to be aligned with the material sample UI and theme we were using for the flutter HVAC app at , we are applying some small changes to the homescreen and the launcher webapps.

For now we are just using the same color theme and the generated gradient image, but might be interesting to check if there's any thought about some deeper look-and-feel overall renewal.

So far the initial idea we we are doing starting by a simple change on the general aspect of the homescreen and launcher which could be replacing the default background image and a few general styles of the common elements that are being used at the webapps. Then this story can be used as a reference for applying some specific style revamp.
For now we are keeping the structure of the launchers at the left and the main area on the rest of the right view, but this could be also changed. In any case, the we are generating a new stylesheet that overrides styles in the existing demo webapps.