Fix Firefox scrollbar color regression in #9210 (#9908)
Fix Firefox scrollbar color regression in #9210 (#9908)

In #9210 the track color was changed to transparent for no apparent reason. This PR restores the previous color selections implemented in #8653 while keeping the updated property name. Per https://github.com/tootsuite/mastodon/pull/9210#issuecomment-441788776 there is no particular reason for the color change in #9210.
im-in-space/mastodon,primenumber/mastodon,imas/mastodon,Kirishima21/mastodon,pso2club/mastodon,clworld/mastodon,lindwurm/mastodon,tootsuite/mastodon,d6rkaiz/mastodon,kazh98/social.arnip.org,lindwurm/mastodon,kazh98/social.arnip.org,cobodo/mastodon,Nyoho/mastodon,glitch-soc/mastodon,musashino205/mastodon,lynlynlynx/mastodon,musashino205/mastodon,h3zjp/mastodon,tootcafe/mastodon,koba-lab/mastodon,dunn/mastodon,tootcafe/mastodon,salvadorpla/mastodon,gol-cha/mastodon,5thfloor/ichiji-social,cobodo/mastodon,koba-lab/mastodon,theoria24/mastodon,Nyoho/mastodon,salvadorpla/mastodon,pso2club/mastodon,clworld/mastodon,yukimochi/mastodon,tootsuite/mastodon,pso2club/mastodon,abcang/mastodon,d6rkaiz/mastodon,tri-star/mastodon,tateisu/mastodon,masto-donte-com-br/mastodon,kirakiratter/mastodon,yukimochi/mastodon,danhunsaker/mastodon,yi0713/mastodon,sylph-sin-tyaku/mastodon,tootsuite/mastodon,MitarashiDango/mastodon,pinfort/mastodon,theoria24/mastodon,MitarashiDango/mastodon,tri-star/mastodon,glitch-soc/mastodon,5thfloor/ichiji-social,imas/mastodon,abcang/mastodon,yi0713/mastodon,yukimochi/mastodon,kazh98/social.arnip.org,ykzts/mastodon,kirakiratter/mastodon,clworld/mastodon,tootsuite/mastodon,gol-cha/mastodon,gol-cha/mastodon,pso2club/mastodon,dunn/mastodon,ikuradon/mastodon,ikuradon/mastodon,koba-lab/mastodon,danhunsaker/mastodon,maa123/mastodon,rainyday/mastodon,im-in-space/mastodon,rainyday/mastodon,abcang/mastodon,musashino205/mastodon,Kirishima21/mastodon,lindwurm/mastodon,KnzkDev/mastodon,d6rkaiz/mastodon,Ryanaka/mastodon,KnzkDev/mastodon,mimumemo/mastodon,maa123/mastodon,MitarashiDango/mastodon,lynlynlynx/mastodon,im-in-space/mastodon,Ryanaka/mastodon,salvadorpla/mastodon,kazh98/social.arnip.org,MitarashiDango/mastodon,kirakiratter/mastodon,danhunsaker/mastodon,primenumber/mastodon,pinfort/mastodon,Kirishima21/mastodon,ikuradon/mastodon,ykzts/mastodon,sylph-sin-tyaku/mastodon,primenumber/mastodon,theoria24/mastodon,tateisu/mastodon,rainyday/mastodon,masto-donte-com-br/mastodon,primenumber/mastodon,tateisu/mastodon,riku6460/chikuwagoddon,tri-star/mastodon,dunn/mastodon,Kirishima21/mastodon,tri-star/mastodon,pinfort/mastodon,tootcafe/mastodon,riku6460/chikuwagoddon,imas/mastodon,h3zjp/mastodon,kirakiratter/mastodon,tootcafe/mastodon,koba-lab/mastodon,cobodo/mastodon,masto-donte-com-br/mastodon,h3zjp/mastodon,ykzts/mastodon,glitch-soc/mastodon,lindwurm/mastodon,masto-donte-com-br/mastodon,KnzkDev/mastodon,maa123/mastodon,Nyoho/mastodon,KnzkDev/mastodon,pinfort/mastodon,yi0713/mastodon,lynlynlynx/mastodon,clworld/mastodon,dunn/mastodon,Ryanaka/mastodon,theoria24/mastodon,5thfloor/ichiji-social,5thfloor/ichiji-social,glitch-soc/mastodon,imas/mastodon,Ryanaka/mastodon,mimumemo/mastodon,maa123/mastodon,gol-cha/mastodon,yi0713/mastodon,ikuradon/mastodon,h3zjp/mastodon,riku6460/chikuwagoddon,yukimochi/mastodon,mimumemo/mastodon,danhunsaker/mastodon,riku6460/chikuwagoddon,ykzts/mastodon,cobodo/mastodon,rainyday/mastodon,im-in-space/mastodon,lynlynlynx/mastodon,d6rkaiz/mastodon,sylph-sin-tyaku/mastodon,abcang/mastodon,sylph-sin-tyaku/mastodon,musashino205/mastodon,tateisu/mastodon,mimumemo/mastodon,Nyoho/mastodon,salvadorpla/mastodon