Use parts to allow customizing styles

I would like to be able to control the styling of web components better, so that they are more in line with the rest of the design when necessary. For example: I'm using the `gmpx-place-picker` in a bigger form, and there's currently no way to change the input border.

A solution would be to use the [part selector](https://developer.mozilla.org/en-US/docs/Web/CSS/::part) to target inner element when necessary.

An alternative would be to use more variables and less hardcoded values in the css and overwrite them. But I'm afraid that will either not be sufficiently detailed enough or bloat the code too much.