Theme: semantic changes and visbility improvements

-  .installed-icon: indicates that an app is installed on your system, it is not a selection. Thus it should use the theme's success_color (in case of Adwaita it's green), which this commit changes
- .counter-label: indicates that X apps are beeing installed or X updates are availible. Both indications should also be done with the success color since it is not a selection and it is positive to have updates, which this commit changes
- .star-enabled: the star is basically invisible in the dark Adwaita theme, since the shade function currently used blends the star too much with the background in the dark theme, changing this to simply fg_color is perfectly visible in both dark and light Adwaita themes
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