Update signal plots to comply with CMS CAT guidelines

[deriggif]

The signal plots need to be updated to comply with the CMS CAT guidelines. In particular, the plotSignalModel() function should be revised to:

• adopt the new color scheme
• add the CMS luminosity label
• increase label and text sizes

and generally improve readability by making the plots less visually heavy.
This update is needed to ensure consistency with current CMS presentation standards.