Try
[{"id":"d52013405d3454db","type":"ui_template","z":"e178618d12a06b0e","group":"a3fa2dec44535f8f","name":"","order":69,"width":0,"height":0,"format":"<style>\n \n #nr-dashboard > div.md-dialog-container > md-dialog > md-dialog-actions > button {\n background-color: rgb(0, 250, 0) !important;\n /* add your custom css here */\n border-radius: 15px\n }\n\n </style","storeOutMessages":false,"fwdInMessages":false,"resendOnRefresh":true,"templateScope":"global","className":"","x":760,"y":380,"wires":[[]]},{"id":"a3fa2dec44535f8f","type":"ui_group","name":"Monitoramento de vida útil","tab":"e92507140fa94b1e","order":1,"disp":true,"width":"30","collapse":false,"className":""},{"id":"e92507140fa94b1e","type":"ui_tab","name":"EEEEEEEE","icon":"dashboard","disabled":false,"hidden":false}]
#nr-dashboard > div.md-dialog-container > md-dialog > md-dialog-actions > button {
background-color: rgb(0, 250, 0) !important;
border-radius: 15px
}
Add to site head, not widget in group
A forum search would have given Search results for 'style notification button' - Node-RED Forum
Then first result Style notification buttons - #4 by Steve-Mcl