Compact fix for color generation in theme.json

[codewithfeeling]

While testing I found that the themeContent CSS sometimes does not contain a closing curly bracket, meaning the index of the last bracket was -1. This cut off the final --color-example: #abcdef definition by one character, leading to the generated theme.json color for that one to be:

{
          "name": "example",
          "slug": "example",
          "color": "#abcde"
 }

By checking for the presence of the final bracket, we can either slice to there, or to the end of the content if it is not present.

const endIndex = themeContent.lastIndexOf('}');
const rootContent = themeContent.slice(themeContent.indexOf('{') + 1, endIndex === -1 ? undefined : endIndex);

[retlehs]

Thank you!