Make content in tooltip better visual experience.

[timoyan]

Take different color in different theme (dark and light).

Current is really not very clear in light mode because font color and background color are too simliar.

[CLAassistant]

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[timoyan]

Btw, I try to run Run linter/prettier: npx grafana-toolkit plugin:dev according to https://github.com/panodata/grafana-map-panel/blob/develop/CONTRIBUTING.md
Found there is 9 places need to be fixed which not belongs to this pull request change, should I submit?

[amotl]

Dear Timo,

thanks for your contribution. Our fork grafana-map-panel includes the improvement #191 by @dtheb (see also #169), amongst many others. See also #281.

With kind regards,
Andreas.