For the vast majority of Siebel implementations which rather focus on desktop-based deployment, the Tab-based Aurora theme will be the most convenient and efficient theme to use. Although the Synergy (or “simplified” UI) theme definitely looks way more sexy, if it comes down to end-user efficiency for typical call center use cases, efficiency is what matters most.
While test driving Siebel 15.0 these days, I found the Thread bar to be missing. That must have been an error wile tweaking the Aurora theme I guess. Although the Aurora theme has not undergone structural changes between IP14 and Siebel 15, they have been re-generated. Development uses LESS to generate the CSS.
Going into Google’s developer tools, the root-cause was identified rather quickly. The Siebel Toolbar was hiding it. A simple tweak to the Aurora CSS was enough as quick fix it. That said, any theme overrides should be done in an custom style sheet. Not in the out-of-the-box theme-aurora.css, which will be likely overwritten by a patch set.
Creating custom override rules as such, is extremely simple with Innovation Pack 14 onwards. Just create a custom css in the /files/custom folder. Administer it as Application / Theme / Platform Independent. Just make sure that it will load after the standard (theme-aurora.css, theme-sui.css, …) theme gets loaded. Because otherwise, well per ‘cascading style sheet’ principle, the custom CSS rules with not override the standard ones!
Next will have to do the necessary and raise a Bug.