Added description of the new theme system

Theme-system.md

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+There is a Theme system in PP, so each app can use the same color scheme, that can be changed by the user.
+
+## Using theme system
+
+It is important to use this system to give users consistent UI. Have to mention there are use cases when you must use specific colors, like in a waterfall, then do it.
+
+To use the theme system, just include **theme.hpp**, and instead of using Color::grey() or similar colors, use the corresponding theme object.
+For example the labels usually used LightGrey color. Now you can use the template's light foreground color. 
+``` Theme::getInstance()->fg_light ```
+
+
+The theme system uses (mostly) styles, that starts with fg_ or bg_ . 
+These can be light, medium, dark, darker, darkest.
+FG stands for foreground, BG stands for background. This is to specify if the foreground is important to you, or the background. So if it is important, that the foreground of a button be light, then use the fg_light. then the foreground will be light colored (like light grey) but the background for it will be something that has good readability.
+
+There are specific colors to indicate "error", "warning", or "ok". These are: error_dark; warning_dark; ok_dark;
+There are also specific color styles, like fg_red, fg_green, fg_yellow, fg_orange, ....
+Use these, because theme can override the color of the red, so within a "red theme" (where the backgrounds are red too) the 255,0,0 color could be different for better contrast. But if you use Color::red() it may not be as readable as the theme's fg_red.
+
+Also if you use theme specific colors, like fg_red->foreground, you should use it's background counterpart. (in the example it would be fg_red->background).
+
+
+The app's background is usually bg_darkest->background.
+
+
+You don't need to use the styles as a style object! For labels you can specify the foreground color only. 
+``` Theme::getInstance()->fg_light->foreground ```  or for the app or widget's background you can use ``` Theme::getInstance()->bg_darkest->background ```
+
+
+## Creating a new theme
+Step 1: open theme.hpp and add a new class derived from ThemeTemplate:
+``` 
+class ThemeMyNewTheme : public ThemeTemplate {
+   public:
+    ThemeMyNewTheme ();
+};
+```
+
+Step 2: add it to the enum (ThemeId) within the Theme class, BEFORE MAX.
+``` 
+enum ThemeId {
+        DefaultGrey = 0,
+        Yellow = 1,
+        Aqua = 2,
+        Green = 3,
+        Red = 4,
+        MyNewTheme = 5,
+        MAX
+    };
+```
+Step 3: In the theme.cpp create a case block for your new enum and class in the Theme::SetTheme() function.
+```
+void Theme::SetTheme(ThemeId theme) {
+    if (current != nullptr) delete current;
+    switch (theme) {
+        case MyNewTheme :
+            current = new ThemeMyNewTheme ();
+            break;
+        ....
+        case DefaultGrey:
+        default:
+            current = new ThemeDefault();
+            break;
+    }
+}
+```
+Step 4: Implement your class's constructor, and create each element! If you forget one, then your theme will crash the PP!
+Best practice is to copy an exists one and modify the color defined in it.
+
+
+Step 5: Always check your theme everywhere. Open each app, and look for missing or unreadable texts.