GTK-3 UI improvements

[GitUser-IamALIVE]

fixed visibility of text , fixed looks , fixed margins of bottom buttons to fit the theme and layout
fixed cachyos using gtk 3 light theme by default and not using dark-theme even when theming is set to dark.

1. dark one is how it looks after pr
2. light one is how it looks ( its hard coded light for some reason ( dosent follow the dark theme )

[vnepogodin]

Don't set custom theme, and don't add don't comments

[GitUser-IamALIVE]

1. "Don't set custom theme" :
   i did not set a custom theme :
   " // Set dark theme preference
   if let Some(settings) = gtk::Settings::default() {
   settings.set_property("gtk-application-prefer-dark-theme", true);
   }
   "

this line just adds a gtk-3 property that make it use adw-dark instead of adw ( gtk-3 doesnt respect gsettings ) , this makes it looks closer the how cachyos-hello is supposed to

" background-color: #1d1d1d; /* Darker than default Adwaita dark */
color: #e0e0e0;"

this is same colour as what cachyos-hello looks like in kde , i use it to make it look consistent over desktops

concerns that you might have :

1. is this theme no over writable by user themes : no this theme can easily be changed by user applied theme

2. also i will remove the comments

[GitUser-IamALIVE]

3. also fixing the hardcoded background