I think preview mode follows the normal code rules and is selected with .markdown-preview-view code
.
This is what I use for the editor:
/* Inline code in the editor */
.cm-s-obsidian span.cm-inline-code,
.cm-s-obsidian span.cm-inline-code:not(.cm-formatting):not(.cm-hmd-indented-code):not(.obsidian-search-match-highlight) {
color: RED ;background-color: BLUE;
}
Change RED and BLUE to suit.