Tiki 25 is a kind of milestone release with many significant new features and revamps and upgrades of existing features and technologies such as the migration from Bootstrap 4 to 5. Regarding themes, here are some changes.
Two new themes have been added to the themes included with Tiki: Boosted and Neubrutiki. More information about these will be added to this page soon.
The Ohia theme was thoroughly overhauled so that its color palette is less extreme. From the beginning its colors were based on what you see around Ohia trees in Hawaii but this wasn't really done well originally. The new palette is a better representation, dominated by the gray color of cooled lava (ohia trees are among the first plants to take root after lava has cooled.
The Darkshine theme is probably easier to configure now, regarding its top and topbar module zones. More info shortly.
Both of the Strasa child themes, Cool and Mono, have been retired, as have been all of the Jqui child themes except for Trontastic. They were removed because they didn't contribute much to the variety of included Tiki themes. For the most part, they could be replicated by using CSS variables in Look & Feel admin CSS customization. Jqui's Excitebike theme was more interesting graphically but wasn't close to meeting WCAG color contrast minimums and if the time was going to be spent fixing it, it would probably be better to devote the time to a new, more contemporary theme.
For child themes that have a significant amount of color variation in elements like buttons, relative to the parent theme and/or "sibling" themes, now Bootstrap SCSS partial files are used, similar to in a standard theme, rather than copying and modifying CSS. This is a more robust method and makes theme maintenance easier. Fivealive, Fivealive-lite, and Jqui child themes now use this method.
New in Tiki 25 is the ability to use CSS variables to easily change the background and foreground colors, and site title font sizes, in the top and topbar zones, where typically navbars are used. There are also CSS variables for the new Unified Admin Backend navbar and aside menu elements, and for pagetop padding. See CSS Variables in Tiki for more details.
Previously, there were dark and light options for navbars, and these were set for every navbar. With the new CSS variables, it's possible to easily specify one set of dark foreground and background colors for the top navbar and another set for the topbar navbars. See Pagetop Styles and CSS Variables for more information.
As themes were updated for Bootstrap 5, they were checked to see if they met color contrast standards, and edited accordingly if they didn't. Compliance isn't 100%, because much depends on the particular HTML and page elements and so on being tested, but there's been considerable improvement. More specifics are coming soon.