[Fleet][EUI Visual refresh] Remove custom colors from code and replace them with EUI color tokens

[criamico]

Context

The EUI team is introducing a new theme called "Borealis". As part of this change we need to update some colors in Fleet. See parent issue: https://github.com/elastic/kibana/issues/199715 - it also contains the instructions to run the new theme (see section Instructions for updating and QA)

Checklist

• [ ] Replace any custom colors across Fleet with EUI color tokens, either in code or inside css styling. Details

:warning: Some occurrences across Fleet:

• https://github.com/elastic/kibana/blob/87af12d21def2ff03e47d0df59b28dfa11436fa8/x-pack/plugins/fleet/public/applications/fleet/sections/agents/agent_list_page/components/agent_activity_flyout/helpers.tsx#L13
• https://github.com/elastic/kibana/blob/87af12d21def2ff03e47d0df59b28dfa11436fa8/x-pack/plugins/fleet/public/applications/integrations/sections/epm/components/package_card.tsx#L50
• https://github.com/elastic/kibana/blob/87af12d21def2ff03e47d0df59b28dfa11436fa8/x-pack/plugins/fleet/public/applications/fleet/sections/agents/agent_requirements_page/es_requirements_page.tsx#L53
• https://github.com/criamico/kibana/blob/8c14ca26ecce910f779ea7c050834014db3817dc/x-pack/plugins/fleet/public/applications/fleet/sections/agent_policy/create_package_policy_page/multi_page_layout/components/horizontal_page_steps.tsx#L32
• https://github.com/criamico/kibana/blob/8c14ca26ecce910f779ea7c050834014db3817dc/x-pack/plugins/fleet/public/applications/fleet/sections/agent_policy/create_package_policy_page/multi_page_layout/components/horizontal_page_steps.tsx#L18
• We need to verify these as they seem to be used for D3 visualization charts: https://github.com/elastic/kibana/blob/87af12d21def2ff03e47d0df59b28dfa11436fa8/x-pack/plugins/fleet/public/applications/fleet/sections/agents/components/donut_chart.tsx#L35

Definitions of done

• [ ] When testing the changes, test them both with the "old" Amsterdam theme and with the new Borealis theme.
• [ ] Please label the PR with EUI Visual Refresh for cross-team visibility

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)