ui: introduce some variation in element background colors

[buck54321]

This is an alternative to #2588. Some features

• As in ui: Dialog panel coloring #2588, this introduces some new colors to break the visual monotony of the current layout.
• The header is a slightly darker color in light mode and a slightly lighter color in dark mode.
• Form elements get the same treatment, along with drop shadows. In dark mode, form elements get a light border.
• The #forms element gets just a little background transparency.
• A distinction is made between the init / login / register view, where the header is not hidden by the #forms element, and all other views, where it is.
• It may not be immediately noticeable, but the background color in light mode is tuned down just a bit. Pure white was kind of aggressive.
• New colors are used sparingly in non-form views.

Registration / Login

Forms on the wallets view

Wallets

Markets

[peterzen]

Registration / Login

The modals look very nice and polished.

Markets

The borders don't represent the visual hierarchy; the lines between the 4 main columns have the same appearance as the grid in the charts, the separators in the left and right columns and even the border of the asset search field. If they are meant to represent the grouping of the elements a few different colors/widths would have to be used.

[peterzen]

Maybe take the opacity of the inactive tab down a notch?

[peterzen]

On a more general note, not strictly within the scope of this PR but related - the main call to action submit buttons (Create BTC wallet, Create account) could use some accent, i.e. background color.

The subsection titles and the buttons have similar appearance.