Add default grid color option to match dark and light color themes.

[MosaicMan]

This adds the ability to specify a default gridColor for cells without matching data so the styling can better suit themes with light or dark colors. Attempts to address #6.

const calendarData = { 
    entries: [],
    year: 2022,
        gridColor: "#2a2a2a", // (optional) Defaults to "#ffffff"
}

[asphaltbuffet]

eagerly awaiting this to be included!

[Richardsl]

i think we should keep the styling separate in custom css snippets. i gave all of the empty days a separate class isEmpty now, so they should be easy to style

[MosaicMan]

Way better solution. Thanks!