[APM] Support EUI lightmode

[formgeist]

To support the official EUI lightmode the variables in https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/public/style/variables.ts should be replaced with the official EUI variables, and the code base should only use these.

To import the light mode

import euiLightVars from '@elastic/eui/dist/eui_theme_light.json';

[elasticmachine]

Pinging @elastic/apm-ui

[snide]

The Infra UI team has a very similar setup to APM with styled components. If you want to see an example of how they're handling it you can check it out over here. Essentially just passing down the theme as a context and loading the EUI JSON vars file as needed. #29133

[formgeist]

@jasonrhodes fyi I merged my refactor of the colour variables we use in APM UI and replaced with the EUI light theme. #29822

[jasonrhodes]

Cool, sounds like this ticket can be closed then right @formgeist ?

[formgeist]

@jasonrhodes Yup, I'm fine with closing it. There's still a few custom colours left that's used for specific components of ours, which I think it's fine to leave in for now. We may have to revisit at a later point.