`<tt>` has been replaced with `<span class="tt">` in latest doxygen

Suggested CSS:

```css
span.tt {
    display: inline;
    background: var(--code-background);
    color: var(--code-foreground);
    padding: 2px 6px;
    font-family: var(--font-family-monospace);
    /*font-size: var(--code-font-size) !important;*/
    border-radius: var(--border-radius-small);
    border: 1px solid var(--separator-color);
    overflow: hidden;
}
```