Hi @Aisutera
Assuming that you have a reasonably up to date version of Node-red, you can do it like this:
Send a message to the text input which has msg.className set - eg msg.className = "red"
Use a template node to define css applicable to <div class="nr-dashboard-textinput red">
[{"id":"ee097897554f457d","type":"ui_text_input","z":"2bfc647470de92f1","name":"","label":"","tooltip":"","group":"c6801e74081486d4","order":1,"width":0,"height":0,"passthru":false,"mode":"text","delay":300,"topic":"topic","sendOnBlur":true,"className":"","topicType":"msg","x":460,"y":120,"wires":[[]]},{"id":"ea7d78e960299ecc","type":"ui_template","z":"2bfc647470de92f1","group":"c6801e74081486d4","name":"CSS styles","order":1,"width":0,"height":0,"format":"<style>\n.nr-dashboard-textinput.red {\nborder: 1px solid red;\n}\n.nr-dashboard-textinput.green {\nborder: 1px solid green;\n}\n</style>","storeOutMessages":true,"fwdInMessages":true,"resendOnRefresh":true,"templateScope":"local","className":"","x":110,"y":60,"wires":[[]]},{"id":"f11835123a2d4b37","type":"ui_button","z":"2bfc647470de92f1","name":"","group":"c6801e74081486d4","order":2,"width":0,"height":0,"passthru":false,"label":"red","tooltip":"","color":"","bgcolor":"","className":"","icon":"","payload":"red","payloadType":"str","topic":"topic","topicType":"msg","x":90,"y":100,"wires":[["fbe9346917a26635"]]},{"id":"d3dbefb3996c987f","type":"ui_button","z":"2bfc647470de92f1","name":"","group":"c6801e74081486d4","order":2,"width":0,"height":0,"passthru":false,"label":"green","tooltip":"","color":"","bgcolor":"","className":"","icon":"","payload":"green","payloadType":"str","topic":"topic","topicType":"msg","x":90,"y":140,"wires":[["fbe9346917a26635"]]},{"id":"fbe9346917a26635","type":"change","z":"2bfc647470de92f1","name":"","rules":[{"t":"move","p":"payload","pt":"msg","to":"className","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":270,"y":120,"wires":[["ee097897554f457d"]]},{"id":"c6801e74081486d4","type":"ui_group","name":"Default","tab":"da2aec15ed6c8eb2","order":1,"disp":true,"width":"6","collapse":false,"className":""},{"id":"da2aec15ed6c8eb2","type":"ui_tab","name":"Home","icon":"dashboard","disabled":false,"hidden":false}]