Dark/Light Theme

1. Add something like light & dark mode. This dark mode is implemented by adding a dark layer above the image when activated, preferably using nord0 or similar color, while the light mode is removing (or just changing the opacity to 0) the darkener layer. I choose to use Nord theme since we are in-line with what Nord theme provides. Nord theme is basically a set of color palette with arctic & north-bluish theme (maybe we need to put some information about it as well in the README.md. Winterly Tab & Nord theme would be a very icy & good match. More about Nord theme can be read in [nordtheme.com](http://nordtheme.com/).
2. For the light and dark mode, we can use the device default theme as the default winter tab theme. A toggle button would be also good for switching between themes.