add adwaita light theme

[AbrA-K]

Hey, I've been using the adwaita-dark theme and would love to see a adwaita-light theme when in a bright environment.
I've started to play around with the dark theme and made a light version. I tried to stay as close to the original adwaita-dark theme, but I made some changes:

• split_and_borders was changed from #C6C6C6 to #4F4F4F
• added blue_0 #d3e4f9 which lighter than the existing blue_1 and is for selections
• picked the darker version of colors to improve contrast (i.e., the adwaita-dark constructor is blue_2 and in my light version it's blue_4)
• The ui.background is #FFFFFF. On one hand, I don't like a full white background. On the other, it's what gnome/libadwaita uses a lot for the light mode and it looks the best in gnome-console (kgx).
• ui.cursor.match in dark mode has a blue background and a white foreground. I swapped them (white/grey-ish backround and blue foreground) because of personal preference. Could be changed - I personally just thought it was easier to distinguish between the cursor and the match.
• ui.bufferline.active has a bold modifier - personal preference again

Here are some screenshots:

in comparison, here is the dark version:

A pull request would be more fitting, but I've heard it's better to ask if this addition is wanted. So, what do you all think?

[AbrA-K]

wow I just realized that the color palette in gtksourceview5 for Adwaita are in fact different from the colors in Adwaita-Dark..🤦 well, I'll change that later and wait for the verdict.