The MediaWiki developers have been slowly improving the accessibility of the user interface. The next step in this transition will change the appearance of some buttons and may break some outdated (non-updated or unmaintained) user scripts and gadgets.
You can see and use the old and new versions now. Most editors will only notice that some buttons are slightly larger and have different colors.
Comparison of old and new styles
Buttons before the change
Buttons after the change
However, this change also affects some user scripts and gadgets. Unfortunately, some of them may not work well in the new system. If you maintain any user scripts or gadgets that are used for editing, please see mw:Contributors/Projects/Accessible editing buttons for information on how to test and fix your scripts. Outdated scripts can be tested and fixed now.