That’s a lot of questions at once. I’ll try to break them down and answer them:
Menu
As you’ve already noticed, menu management in Block Themes has changed fundamentally. In future, the menu will be managed within the Site Editor. You can find an introduction to this here: https://learn.wordpress.org/tutorial/how-to-create-a-menu-with-the-navigation-block/
Alternatively, there are plugins that restore the classic themes. This would allow you to maintain the menu as before. One example of this would be https://wordpress.org/plugins/classic-menus-for-block-themes/ – however, there are a few potential hurdles here: pay close attention to the instructions on how to integrate the classic menu into the block theme. In addition, this will only work as long as there are no major changes to the menus in block themes.
This is because you cannot use it to build mega menus, for example. Currently, the best way to do this in block themes is with the Olli Menu Designer plugin (https://wordpress.org/plugins/ollie-menu-designer/), whose integration into the WordPress core is already being discussed.
Colors
There probably isn’t a plugin that can change all colors in a theme at once. With block themes, this isn’t necessary anyway. Here, you can (unlike with most classic themes) set your base colors once and assign them to the respective components (text, buttons, links, etc.). If you then want to change a color, you change it once in the global settings and it changes immediately everywhere. You can find instructions for this here: https://learn.wordpress.org/tutorial/how-to-style-your-site-with-global-styles/
With classic themes, you would need to contact their support team to find out what options are available. Often, changes to the supplied CSS file are necessary, but not always.
TwentyTwentySix Theme
As far as I know, the community is not currently working on a new standard theme. On the contrary, there are no plans to create any more. However, this is currently being discussed among members of the open source community for WordPress. You can find out more about this at any time by following https://make.wordpress.org/core/.
