Improve the style of the summary plots

The summary plots of VetoPerf are difficult to read. The vetoed triggers, currently marked with a cross, are difficult to see. For example: https://ldas-jobs.ligo-wa.caltech.edu/~detchar.upv/NewUPV/O4/results/rolling_weekly/daily_vp/hierarchical/O4-H-1397606418_1397692818/vp/H1-GDS_CALIB_STRAIN-vetoall_freqtime.png

Ideas to improve the style:

• Use plain black circle for the triggers
• Use color markers instead of crosses
• Revert the color scale: use the red color for the winning channel of round 1 (instead of blue)

cc @nelson-christensen @alex.lattal @jay.tasson @biancarose.lott