I'd rather rely on the room lighting to keep my pupils small. I don't like staring directly at bright light sources such as lamps, the sun, or white editor backgrounds 
1) Which means, your eyes need to adjust when you switch from looking at the screen and elsewhere.
2) light theme is in
no way bright light. Set up your monitors brightness (or contrast if it does not have brightness) correctly.
As I said, many monitors come with very bright defaults. That doesn't mean they can't be used with less.
Depending on your monitor, the settings may be between 50% and 75% of the max brightness. And yes, for the first 5 to 10 minutes your brain will tell you that the white background is now grey. Your brain should adjust its perception rather quickly.
But yes, the room lightening is also important.
As a correct monitor settings. Wrong sub-pixel settings together with anti aliasing, dpi scaling (or an editor that uses sub-pixel resolution / Lazarus does not) can blur the display. And depending on the mix of your screen type and the selected wrong setup, this may manifest differently depending on which colors are in use (and light vs dark changes those colors).
Also unless you paid several $1000 your screens colors may not be accurate. This means that contrast between default color settings may be different between different screen hardware. For some screens therefore it may be needed to change some of the supplied colors (e.g. make blue strings darker/brighter or slightly change the amount of other base colors mixed into that blue. (But that can happen for dark themes too)
Of course if you have an extremely bad monitor, and the white of surrounding pixels bleeds into the glyphs of the text, then a light theme will be a problem. In a dark theme that will just mean the text gets printed "bold"-er /-ish.