override standand css classes for example
.sapMInputBaseInner{
background:red;
}
.sapMComboBoxBaseArrow,.sapMComboBoxBasePressed .sapMComboBoxBaseArrow, html.sap-desktop .sapMComboBoxBasePressed .sapMComboBoxBaseArrow:hover{
background:red;
}
Chrome DevTools Overview - Google Chrome
may be helpful for you to find style information and classes.