Looks like the CSS was slightly wrong, this below seems to work OK:
.custom-control-input {
background-color: #6C757D;
}
Updated Razor/HMTL:
<div class="mb-3" style="margin-top: 1.5em">
<div class="d-flex flex-row">
<div>
Conditional Logic:
</div>
<div class="form-check form-check-inline ms-2">
<input checked class="custom-control-input form-check-input" type="radio" name="RadioBtn1" id="customRadio1" value="OR">
<label class="form-check-label" for="customRadio1">OR</label>
</div>
<div class="form-check form-check-inline">
<input class="custom-control-input form-check-input" type="radio" name="RadioBtn1" id="customRadio2" value="AND">
<label class="form-check-label" for="customRadio1">AND</label>
</div>
</div>
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