Since NewXkit's Old Blue is failing in places here's my simple version - I Might update it to be customisable later on!
Tumblr's Old Blue by Pixiel

Details
AuthorPixiel
LicenseNo License
Categoryhttps://www.tumblr.com/
Created
Updated
Code size4.0 kB
Code checksum3499f418
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Description
Notes
Works with most Tumblr Color Palettes except Vampire and Pumpkin - which for some reason turns the post icons green
Source code
/* ==UserStyle==
@name Tumblr Old Blue
@namespace github.com/openstyles/stylus
@version 1.4
@description Since NewXkit's Old Blue is failing in places heres my simple version - Best used with with Tumblrs "True Blue" color palette
@author Pixiel
@var select old-new-font "Old or New font" {"Old": "Helvetica Neue, HelveticaNeue, Helvetica, Arial, sans-serif","New": "Favorit"}
==/UserStyle== */
@-moz-document url-prefix("https://www.tumblr.com/") {
:root {
--rgb-white: 255, 255, 255;
--rgb-white-on-dark: 191, 191, 191 !important;
--rgb-black: 68, 68, 68;
--navy: 54, 70, 93;
--red: 217, 94, 64;
--orange: 242, 153, 46;
--yellow: 232, 215, 56;
--green: 86, 188, 138;
--blue: 82, 158, 204;
--purple: 167, 125, 194;
--pink: 116, 128, 137;
--secondary-accent: 229, 231, 234;
--follow: 243, 248, 251;
--white: var(--rgb-white);
--white-on-dark: var(--rgb-white-on-dark);
--black: var(--rgb-black);
--transparent-white-65: rgba(var(--rgb-white-on-dark), 0.65);
--transparent-white-40: rgba(var(--rgb-white-on-dark), 0.4);
--transparent-white-25: rgba(var(--rgb-white-on-dark), 0.25);
--transparent-white-13: rgba(var(--rgb-white-on-dark), 0.13);
--transparent-white-7: rgba(var(--rgb-white-on-dark), 0.07);
--gray-65: rgba(var(--rgb-black), 0.65);
--gray-40: rgba(var(--rgb-black), 0.4);
--gray-25: rgba(var(--rgb-black), 0.25);
--gray-13: rgba(var(--rgb-black), 0.13);
--gray-7: rgba(var(--rgb-black), 0.07);
--color-primary-link: rgba(82, 158, 204);
--accent: rgba(82, 158, 204);
--accent-hover: rgba(82, 158, 204);
--accent-pressed: rgba(82, 158, 204);
--deprecated-accent: 82, 158, 204;
--chrome-ui: rgba(82, 158, 204);
--chrome-ui-hover: rgba(82, 158, 204);
--chrome-ui-pressed: rgba(82, 158, 204);
--chrome-ui-focus: rgba(82, 158, 204);
--badge-text: #36465D !important;
--chrome-ui-fg: #36465D !important;
--chrome: #36465D !important;
--chrome-panel: rgb(191, 191, 191, 0.07) !important;
--font-family: var(--old-new-font) !important;
--font-family-modern: var(--old-new-font) !important;
--chrome-fg-secondary: rgba(153, 163, 174, 1);
--chrome-fg-tertiary: rgb(191 191 191 / 65%) !important;
}
Div.DCCfo {
--font-family: var(--old-new-font) !important;
--font-family-modern: var(--old-new-font) !important;
--black: 68, 68, 68 !important;}
Div.DCCfo.ZmMA3 {
--deprecated-accent: 82, 158, 204 !important;
--blue: 82, 158, 204 !important;
}
.k31gt {
color: #000;
}
.s3_eW, .kRc5f {color: RGB(var(--white-on-dark)) !important}
.pPuiK {
background-color: rgb(191, 191, 191, 0.07);
border: 1px solid var(--chrome-panel-border); }
.So6RQ.YSitt > .ge_yK > article > div > .jn3oa, .So6RQ.YSitt > .ge_yK > .c79Av > article > div > .jn3oa {
font-size: 0.67rem;
}
.Ks9nn {background: var(--chrome)}
.rGtDT.jBW90:has([aria-label="Go to community"]) > .UwXX0 > .JI41B > .RFsN3 > .s3_eW {
color: var(--content-fg) !important;
}
.palette--lowContrastClassic .vli36 a>svg, .palette--cement .vli36 a>svg, .palette--cybernetic .vli36 a>svg, .palette--canary .vli36 a>svg, .palette--ghost .vli36 a>svg, .palette--vampire .vli36 a>svg, .palette--pumpkin .vli36 a>svg, .palette--snowBright .vli36 a>svg, .palette--gothRave .vli36 a>svg {
--icon-color-primary: RGB(var(--rgb-white-on-dark));
}
.palette--darkMode .FOqaP, .palette--cement .FOqaP, .palette--lowContrastClassic .FOqaP, .palette--cybernetic .FOqaP, .palette--canary .FOqaP, .palette--ghost .FOqaP, .palette--vampire .FOqaP, .palette--pumpkin .FOqaP, .palette--snowBright .FOqaP {
color: RGB(var(--black));
}
}