External tile maps will not load on dashboard if dashboard is set to dark

[arenzana]

Kibana version: 6.3.0

**Elasticsearch version: 6.3.0 **

Server OS version: CentOS 7.4

Browser version: Firefox 61

Browser OS version: MacOS 10.14

Original install method (e.g. download page, yum, from source, etc.): RPM download

Describe the bug: If I add an external tile map to a visualization in Kibana, it will load correctly. (ie. http://basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png as WMS server). I can now create a Dashboard and the dark map will display correctly. However, if I set the dashboard:defaultDarkTheme option to true and create a new dashboard (it should be dark), the tile map will not load. Perhaps because Kibana fails to change the color set?

Steps to reproduce:

1. Set the dashboard:defaultDarkTheme to true.
2. Create a Visualization that includes a custom WMS server.
3. Create a dashboard (dark) that includes the previously created visualization.

Expected behavior: I would expect the tile map colors to not be changed but load

**Screenshots (if relevant):

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Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context: I'm using Dark Mode in Mojave, but I don't think that's relevant, really.

[nreese]

The tiles are getting loaded. If you open your browser's dev tools and look under network requests, you can see that the tiles are loaded.

The problem is that in dark-mode, a CSS filter is applied to the tile to invert the colors. This makes your tiles show up as all white.

Disabling the CSS rule fixes the issue

You could remove this line from the CSS https://github.com/elastic/kibana/blob/6.3/src/core_plugins/tile_map/public/styles/_tilemap.less#L196 under the source directory and then remove the optimize folder. Restart kibana and optimize will be re-generated from the source files.

Or just use a light tile set and let the filter invert their color.

[thomasneirynck]

related to #18657.

[thomasneirynck]

This is addressed in 7.2. Dark-themed maps are now a first-level tile-service in EMS and Kibana no longer uses a custom CSS rule to manipulate the colors of the tile images.