WIP: Add theming and month abbreviation

[christophbaur]

Work in Progress // yet not testet on mentioned hardware in README.md:

• Add the possibility to change between a dark and a light theme/style.
  • Just copy the files (.css/.png) within render/stlye_light-folder or render/style_dark-folder
  • replace the existing files (.css/.png) in render-folder to switch between a dark and a light style of the calendar.
• Add the possibility to use month abbreviation like 'JAN' instead of month number like '1' Adjustable via config.json: "isMonthAbbr": true --> month abrreviation like 'JAN' displays "isMonthAbbr": false --> default value, month number like '1' displays

[christophbaur]

... still waiting for hardware ...

[Engel56]

calendar nicht definiert