Add this css code inside the custom.css file to dynamic price rules select box issue.
input[type="text"]:not(.form-control), input[type="email"]:not(.form-control), input[type="url"]:not(.form-control), input[type="date"]:not(.form-control), input[type="password"]:not(.form-control), input[type="search"]:not(.form-control), input[type="tel"]:not(.form-control), input[type="number"]:not(.form-control), select:not(.form-control) {
display: block;
width: 100%;
padding: 5px 10px 5px 10px;
font-size: 1rem;
line-height: 1.25;
color: #495057;
background-color: #fff;
background-image: none;
background-clip: padding-box;
border: 1px solid #e4e4e4;
border-radius: 2px;
transition: border-color ease-in-out 0.15s, box-shadow ease-in-out 0.15s;
}
Provide me a screenshot what exactly you want to change