[Theme] Remove legacy Amsterdam theme

[mgadewoll]

Description

EUI has been using the new "Borealis" theme as default since version 100.0.0 (changelog). Until now, we have been keeping the old "Amsterdam" theme in parity to it's previous state next to "Borealis". This meant that we had to "feature-flag" differences either via tokens on the theme or via conditional code changes.

Keeping "Amsterdam" in parity has started to slow down the updates for "Borealis", as updates become more complicated to consider both themes as their designs diverge further.

Considering that "Amsterdam" is not an active theme for EUI anymore, we want to remove it to simplify the workflow for upcoming UI updates on Borealis.

Note

The last actively used EUI version with "Amsterdam" on Kibana 8.19 usage is 104.0.0-amsterdam.0

Required updates

General

• check if there are any internal projects using "Amsterdam" in latest EUI versions (list of internal EUI users)

EUI

• remove packages/eui/src/themes/amsterdam
• remove flags from EuiThemeShape and implementations (code) & cleanup usages of available flags
  • hasGlobalFocusColor
  • hasVisColorAdjustment
  • buttonVariant
  • formVariant
• remove provider override that enables flags triggering recomputation (code)
• remove packages/eui/src/services/theme/theme_variant.ts and cleanup usages of isEuiThemeRefreshVariant and useEuiThemeRefreshVariant (code) ℹ️ We keep this for now, in case we need theme flags for in-code feature flags.

Kibana

• remove dev theme switching functionality (main only)
• remove theme specific conditionals (isAmsterdam, isBorealis) and checks on euiTheme.flags

Cloud-UI

• remove dev theme switching functionality
• remove Amsterdam SCSS loading (code)

Impact/Value

⏩ Maintainability: Removing "Amsterdam" will simplify and speed up UI changes.

Urgency

Considering there are more UI changes coming up (meta), the priority on this is increased to remove the theme to prevent complications on the updates.