I find it a little overwhelming.
Like erikrthoff suggested, I'd drop the light mode toggle and just go with prefers-color-scheme. Then I'd move the X buttons to the top-right of their respective sections (timers, segments), and move the Add Timer button underneath.
Mockup: https://i.imgur.com/EcSHX7S.png
What a great suggestion, and you even made a mockup! That's so considerate of you, thank you so much.
I'll try to implement these UI changes tonight.