v3.0: fix color issues when using _amber theme

[SteffenBrinckmann]

When using ember, the font on the buttons is not that visible. Use real theme.

Notes: qt_material - dark-blue: {'primaryColor': '#448aff', 'primaryLightColor': '#83b9ff', 'secondaryColor': '#232629', 'secondaryLightColor': '#4f5b62', 'secondaryDarkColor': '#31363b', 'primaryTextColor': '#000000', 'secondaryTextColor': '#ffffff'}

TODO:

• get table of all colors
• test with slideShowGui
• from docu: apply_stylesheet( app, theme + '.xml', invert_secondary=('light' in theme and 'dark' not in theme),extra=extra,)