[Infra UI] Fix styling after breaking EUI changes (elastic#27021)

This restores the Infrastructure and Logs UIs after upstream changes:

* The theme json import now behaves like a ES6 module.
* The `<EuiHeaderSection>` now requires the `grow` prop to be `true` in order to grow horizontally.

x-pack/plugins/infra/public/apps/start_app.tsx

@@ -15,7 +15,7 @@ import { ThemeProvider } from 'styled-components';
 
 // TODO use theme provided from parentApp when kibana supports it
 import { EuiErrorBoundary } from '@elastic/eui';
-import * as euiVars from '@elastic/eui/dist/eui_theme_k6_light.json';
+import euiVars from '@elastic/eui/dist/eui_theme_k6_light.json';
 import { InfraFrontendLibs } from '../lib/lib';
 import { PageRouter } from '../routes';
 import { createStore } from '../store';

x-pack/plugins/infra/public/components/header/legacy_header.tsx

@@ -20,7 +20,7 @@ export const LegacyHeader: React.SFC<LegacyHeaderProps> = ({
   breadcrumbs = [],
 }) => (
   <HeaderWrapper>
-    <EuiHeaderSection>
+    <EuiHeaderSection grow>
       <EuiHeaderBreadcrumbs breadcrumbs={breadcrumbs} />
     </EuiHeaderSection>
     {appendSections}

x-pack/plugins/infra/types/eui.d.ts

@@ -39,6 +39,7 @@ declare module '@elastic/eui' {
   export type EuiHeaderSectionSide = 'left' | 'right';
   type EuiHeaderSectionProps = CommonProps & {
     side?: EuiHeaderSectionSide;
+    grow?: boolean;
   };
   export const EuiHeaderSection: React.SFC<EuiHeaderSectionProps>;