I'm far from sure this is the best way to do it but this will change the background hover colour for all buttons
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Or this will change it for a specific button (You have to use your button node's id in the css selector)
[{"id":"44c6a8d6.01ec1","type":"ui_template","z":"6a947c4c.60270c","group":"a6e358b.672ffa8","name":"Button CSS","order":6,"width":0,"height":0,"format":"<style id=\"dashboard-style-override\">\n/*.nr-dashboard-button .md-button:hover {\nbackground-color: #00ffff !important;\n}*/\n[node-id=\"20d45887.d8ef1\"] > button:hover {\n background-color: #ff00ff !important;\n}\n</style>","storeOutMessages":true,"fwdInMessages":true,"resendOnRefresh":true,"templateScope":"local","x":306.20001220703125,"y":435.20001220703125,"wires":[[]]},{"id":"a6e358b.672ffa8","type":"ui_group","name":"Demo","tab":"4e528085.a1bfa","order":1,"disp":true,"width":"20","collapse":false},{"id":"4e528085.a1bfa","type":"ui_tab","name":"Demo","icon":"dashboard","order":2,"disabled":false,"hidden":false}]