Below is what I have for custom checkbox colors with CSS, it could a starting place for you. Put this in a tiddler tagged with $:/tags/Stylesheet and have fun. ^_^
.tc-tiddler-body input[type="checkbox"],
.tc-tiddler-preview-preview input[type="checkbox"],
.tc-sidebar-scrollable input[type="checkbox"] {
-webkit-appearance:none;
background-color: rgba(0,255,255,0.15);
border: none;
box-shadow: 0 0 0 1px rgba(0,255,255,1) inset;
display: inline-block;
height: 0.8em;
outline-color: transparent;
width: 0.8em;
}
.tc-tiddler-body input[type="checkbox"]:checked,
.tc-tiddler-preview-preview input[type="checkbox"]:checked,
.tc-sidebar-scrollable input[type="checkbox"]:checked {
background-color: rgba(0,255,255,1);
}
.tc-tiddler-body input[type="checkbox"]:checked::after,
.tc-tiddler-preview-preview input[type="checkbox"]:checked::after,
.tc-sidebar-scrollable input[type="checkbox"]:checked::after {
color: rgba(238,137,105,1);
content: "\2713";
font-size: 1em;
font-weight: bold;
left: 0;
position: relative;
text-shadow: 1px 1px 1px #000;
top: -0.3em;
}
.tc-tiddler-body input[type="checkbox"]:active,
.tc-tiddler-preview-preview input[type="checkbox"]:active,
.tc-sidebar-scrollable input[type="checkbox"]:active {
border: none;
box-shadow: none;
outline-color: transparent;
}