(await wp.apiFetch({path: 'wp/v2/themes/theme-experiments/tt1-blocks' } ))._links['wp:user-global-styles'][0].href
>> http://yah.local/wp-json/wp/v2/global-styles/33301

await wp.apiFetch({path: 'wp/v2/global-styles/33301', method: 'POST', data: { settings: { color: { palette: [ { color:'red', slug: 'red', name: 'red' } ] } } } } );

 JSON.stringify( await wp.apiFetch({path: 'wp/v2/global-styles/33301'} ) );

>> '{"id":33301,"settings":{"color":{"palette":[{"color":"red","slug":"red","name":"red"}]}},"styles":{},"title":{"raw":"Custom Styles","rendered":"Custom Styles"},"_links":{"self":[{"href":"http://yah.local/wp-json/wp/v2/global-styles/"}],"collection":[{"href":"http://yah.local/wp-json/wp/v2/global-styles"}],"wp:action-publish":[{"href":"http://yah.local/wp-json/wp/v2/global-styles/"}],"curies":[{"name":"wp","href":"https://api.w.org/{rel}","templated":true}]}}'

await wp.apiFetch({path: 'wp/v2/global-styles/33301', method: 'POST', data: { styles: { color: { background: 'green' }  } } } );

JSON.stringify( await wp.apiFetch({path: 'wp/v2/global-styles/33301'} ) );

{"id":33301,"settings":{"color":{"palette":[{"color":"red","slug":"red","name":"red"}]}},"styles":{"color":{"background":"green"}},"title":{"raw":"Custom Styles","rendered":"Custom Styles"},"_links":{"self":[{"href":"http://yah.local/wp-json/wp/v2/global-styles/"}],"collection":[{"href":"http://yah.local/wp-json/wp/v2/global-styles"}],"wp:action-publish":[{"href":"http://yah.local/wp-json/wp/v2/global-styles/"}],"curies":[{"name":"wp","href":"https://api.w.org/{rel}","templated":true}]}}

await wp.apiFetch({path: 'wp/v2/global-styles/33301', method: 'POST', data: { styles: { color: { gradient: 'linear-gradient(135deg,rgb(255,0,0) 0%,rgb(0,0,255) 100%)' }  } } } );

(await wp.apiFetch({path: 'wp/v2/global-styles/themes/theme-experiments/tt1-blocks' } ) );
>> { settings ..., styles ... }