Central theme settings

Add a ThemeProvider to centralize the theming of Vivy.

• No more per widget style with hard-coded colors
• An easy and maintainable theme
• Also add a switch for dark/light theme.

We can use the QtCreator thing, with themes like https://github.com/qt-creator/qt-creator/tree/master/share/qtcreator/themes. The source for parsing and applying those themes are here: https://github.com/qt-creator/qt-creator/tree/master/src/libs/utils/theme.