(fix) use white color for buttons, matching designs

closes #1886 (closed)

• ditch square green buttons they look out of place. All buttons should be replaced with blue text hyperlinks on light mode. Need to confirm the text button color for dark mode. Confirming with Michael

• the buttons in blocked channels have the same square button issue. the post button in the composer with the oval make more sense for now until we bring in the new designs everywhere

• the gallery, photo and video buttons in soon to be legacy composer should not be gray background

NOTE: i fixed the text color for the buttons since in designs the buttons always have white color regardless the theme. I think this make more sense since all the designs will be using this kind of buttons.